This patch does transitional refactoring of string
types in the THD class. This will ease on the migration to
the new strings framework. A number of places in the
server uses char* instead of const char*. A gradual
transformation of the server code is required to use
const char* and use const references to pass objects
around functions. This will enable the smooth transition
of the server code to use the new string framework classes.
    
This changeset is responsible for changing the type of
db name of the Statement class from a raw char* to
LEX_CSTRING. This results in set of cascade changes across
several files. The following data members are modified as
part of this patch:
      1. sp_name
           LEX_CSTRING m_db
      2. Alter_table_ctx
          const char *db
          const char *table_name
          const char *alias
          const char *new_db
          const char *new_alias
      3. Foreign_key
          LEX_CSTRING ref_db
          LEX_CSTRING ref_table
      4. Statement
          LEX_CSTRING m_db
      5. Table_ident
          LEX_CSTRING db
          LEX_CSTRING table
      6. TABLE_LIST
          const char *db, *table_name, *alias
          LEX_CSTRING view_db
          LEX_CSTRING view_name
      7. Trigger
          LEX_CSTRING m_db_name
          LEX_CSTRING m_subject_table_name
      8. user_var_entry
      LEX_CSTRING m_catalog
    
The functions which are modified as part of this patch include:
      1. create_embedded_thd
      2. check_embedded_connection
      3. acl_getroot
      4. check_routine_access
      5. acl_authenticate
      6. check_access
      7. mysql_table_grant
      8. get_table_grant
      9. acl_notify_htons
      10. log_loaded_block
      11. decide_logging_format
      12. net_after_header_psi
      13. Event_job_data::execute
      14. Events::update_event
      15. Events::drop_schema_events
      16. filesort
      17. write_schema_op_to_binlog
      18. injectApplyStatusWriteRow
      19. ndb_binlog_thread_func
      20. handler::ha_external_lock
      21. Item_field::fix_fields
      23. Item_func_sp::itemize
      24. Item_func_database::val_str
      25. File_query_log::write_slow
      26. set_thd_db
      27. Query_log_event::Query_log_event
      28. Query_log_event::do_apply_event
      29. Load_log_event::do_apply_event
      30. Log_event::get_db
      31. handle_slave_io
      32. handle_slave_sql
      33. Current_schema_tracker::store
      34. db_load_routine
      35. lock_db_routines
      36. sp_drop_db_routines
      37. sp_load_for_information_schema
      38. sp_head::execute
      39. sp_head::execute_trigger
      40. sp_instr_stmt::exec_core
      41. mysql_admin_table
      42. Alter_table_ctx::Alter_table_ctx
      43. MYSQL_AUDIT_NOTIFY_CONNECTION_CONNECT (macro)
      44. MYSQL_AUDIT_NOTIFY_CONNECTION_CHANGE_USER (macro)
      45. list_open_tables
      46. open_table
      47. make_cache_key
      48. Query_cache::invalidate
      49. THD::~THD
      50. thd_binlog_filter_ok
      51. db
      52. set_db
      53. reset_db
      54. copy_db_to
      55. Table_ident::Table_ident
      56. Table_ident::change_db
      57. mysql_open_cursor
      58. mysql_change_db_impl
      59. write_to_binlog
      60. get_default_db_collation
      61. mysql_create_db
      62. mysql_alter_db
      63. mysql_rm_db
      64. mysql_change_db_impl
      65. backup_current_db_name
      66. mysql_change_db
      67. mysql_opt_change_db
      68. mysql_upgrade_db
      69. mysql_derived_prepare
      70. Sql_cmd_handler_open::execute
      71. db_is_default_db
      72. mysql_load
      73. write_execute_load_query_log_event
      74. mysql_load
      75. all_tables_not_ok
      76. dispatch_command
      77. mysql_execute_command
      78. mysql_parse
      79. add_table_to_list
      80. plugin_load
      81. Prepared_statement::set_db
      82. Prepared_statement::prepare
      83. Prepared_statement::reprepare
      84. Prepared_statement::swap_prepared_statement
      85. Prepared_statement::execute
      86. mysql_rename_tables
      87. do_rename
      88. close_cached_connection_tables
      89. mysqld_show_create_db
      90. store_create_info
      91. view_store_create_info
      92. List_process_list::operator()
      93. Fill_process_list::operator()
      94. make_table_list
      95. fill_schema_table_by_open
      96. get_all_tables
      97. make_schema_select
      98. get_trigger_table
      99. initialize_information_schema_acl
      100. mysql_rm_table_no_locks
      101. mysql_alter_table
      102. add_table_for_trigger
      103. udf_init
      104. mysql_create_view
      105. mysql_make_view
      106. mysql_drop_view
      107. get_table_category
      108. open_table_def
      109. TABLE_LIST::prepare_view_securety_context
      110. TABLE_LIST::prepare_security
      111. init_one_table
      112. new_nested_join
      113. TABLE_LIST::get_db_name
      114. TABLE_LIST::get_table_name
      115. Table_trigger_dispatcher::Table_trigger_dispatcher
      116. Trigger::create_from_dd
      117. Trigger::execute
      118. Trigger::parse
      119. Trigger::get_db_name
      120. Trigger::get_subject_table_name
      121. Trigger_creation_ctx::create
      122. Trigger_loader::load_triggers
      123. Trigger_loader::drop_all_triggers
      124. my_tz_init
      125. initialize_performance_schema_acl
      126. ParserTest::parse
      127. ACL_internal_schema_registry::register_schema
      128. mysql_routine_grant
      129. Log_to_csv_event_handler::log_slow
      130. sp_exist_routines
      131. create_file
      132. Statement::Statement
      133. end_connection
      134. prepare_new_connection_state
      135. mysql_ha_hash_get_key
      136. init_lex_with_single_table
      137. innobase_get_foreign_key_info
      138. ha_myisammrg::append_create_info
      139. initialize_peformance_schema_acl.
A new helper function to_lex_cstring(const char*) has been
introduced which converts a const char* null-terminated 
string to LEX_CSTRING type.