When a MySQL Server connects to NDB, it does not know whether it has
connected to that cluster before or not. Due to this, it also will not
know if its DD entries are just a little bit out of sync or completely
invalid. This commit introduces a schema version identifier known as
schema UUID to solve this issue. The schema UUID will be a unique
identifier stored in the ndb_schema table in NDB and in the
se_private_data field of the ndb_schema table object in DD.

Whenever a MySQL Server connects with a cluster, it can now compare
the schema UUID stored in its DD against the one stored in the
ndb_schema in NDB and detect if it is connecting for the first time
or not. If it is connecting for the first time, it will wipe clean its
DD entries.

Changes:
ha_ndbcluster_binlog.cc:
 - Updated the method remove_deleted_ndb_tables_from_dd() in
   Ndb_binlog_setup class to optionally wipe clean the DD when an
   initial start/restart is detected.
 - Added method detect_initial_restart() to compare the UUIDs stored in
   DD and NDB, and, detect if this is an initial start/restart or not.
 - Updated the Ndb_binlog_setup class' setup method to detect and
   handle initial system restarts.

ndb_schema_dist_table.h
 - Added a method get_schema_uuid() to retrieve the schema UUID from
   the ndb_schema table in NDB.
 - Overloaded the pre_upgrade() method to save the schema UUID in a
   new static member - old_ndb_schema_uuid.
 - Overloaded the post_install() method to either generate a new
   schema UUID or use the old_ndb_schema_uuid and update them in the
   ndb_schema table after a create/upgrade.
 - Added a method update_schema_uuid_in_NDB() that updates the UUID of
   the ndb_schema table in NDB.

ndb_util_table.h
 - Added a new method pre_upgrade(), that can be overloaded by the
   derived classes to save any additional metadata that has to be
   restored after upgrade.
 - Added a new method post_install(), which can be overloaded by the
   derived classes to update the metadata of the tables further after
   the table has been created in NDB.
 - Added a new method unpack_blob_not_null() to read out the values from
   a blob handle into a buffer.
 - Added get_thd() and get_ndb() methods to be used by the derived
   class methods to access the THD and Ndb objects stored in the object.
 - Updated the return type of pack_varbinary method and its wrapper in
   Ndb_schema_result_table to void.
 - Made the pack_varbinary, unpack_varbinary into protected methods.

ndb_dd_client.h
 - Added methods get_schema_uuid() and update_schema_uuid() to read and
   write the schema UUID values from the ndb_schema table in DD.

ndb_dd_table.h
 - Added methods ndb_dd_table_get_schema_uuid() and
   ndb_dd_table_set_schema_uuid() to read and update
   schema_uuid from and into the ndb_schema table object's
   se_private_data.

ndb_dd.h
 - Added wrapper methods to read and write the schema UUID from and to
   DD.

ndb_thd_ndb.h
 - Added a getter get_thd() for the THD object in the Thd_ndb class.

Updated ndb.ndb_restart and ndb_ddl.schema_dist_upgrade testcases.

Change-Id: Ia82ed94e645615230c02027f606888a550cbcf48