and introduces the following changes:
  * renames the struct st_transactions to Transaction_ctx and
  moves a declaration of Transaction_ctx outside THD class;

  * adds the prefix m_ to all data members of classes THD_TRANS and
  Transaction_ctx;

  * changes the name of data member THD::transaction to THD::m_transaction,
  makes this member as private and provides access to it using the new method
  THD::get_transaction();

  * hides direct access to all data members of the class Transaction_ctx except
  for the data member m_savepoints;

  * adds the methods  init_mem_root_defaults, allocate_memory(), free_memory(),
  strmake() to get access to Transaction_ctx::m_mem_root

  * adds the methods add_changed_table() and
  invalidate_changed_tables_in_cache() into the class Transaction_ctx to give
  access to the data member m_changed_tables

  * removes the data member Transaction_ctx::m_pending_rows_event
  since it is never used within source code.

  * moves declarations/definitions of classes THD_TRANS,
  Ha_trx_info, Transaction_ctx from the files sql_class.h/sql_class.cc to
  the separate files transaction_info.h/transaction_info.cc.

  * introduces transaction_cache and functions to work with it.
  transaction_cache is used instead of xid_cache and stores a pointer to an
  instance of  class Transaction_ctx that contains full information about
  transaction in order to be able to support for detaching/attaching of XA
  transaction from THD in the future;

  * changes the tests setup_instruments_defaults and server_init  since
  the mutex LOCK_xid_cache was replaced by LOCK_transaction_cache and
  corresponding changes should be done in those tests results;

  * adds the method  Transaction_ctx::xid_state() to give access
  to the data memeber Transaction_ctx::m_xid_state;

  * removes the function ha_list_names() and the method
  THD_TRANS::dbug_unsafe_rollback_flags since it is used only in debug log
  output that now is considered as redundant;

  * hides all access to THD_TRANS::m_no_2pc and THD_TRANS::m_rw_ha_count from
  outside by corresponding methods of the class Transaction_ctx -
  set_rw_ha_count()/rw_ha_count(), no_2pc()/set_no_2pc();

  * moves a definition of struct THD_TRANS into the class Transaction_ctx
  to make the class THD_TRANS to be implementaion details of Transaction_ctx;

  * removes save/restore of transaction state from
  Ndb_local_connection::execute_query_iso() as discussed with Magnus Blåudd.