This patch augments Gtid_log_event with the user thread-id.
In particular that compensates for the loss of this info in
Rows_log_events.

Gtid_log_event::thread_id gets visible in mysqlbinlog output like

  #231025 16:21:45 server id 1  end_log_pos 537 CRC32 0x1cf1d963  GTID 0-1-2 ddl thread_id=10

as 64 bit unsigned integer.

While the size of Gtid event has grown by 8-9 bytes
replication from OLD <-> NEW is not affected by it.

This work was started by the late Sujatha Sivakumar.
Brandon Nesterenko took it over, reviewed initial patches and extended
the work.

Reviewed-by: <andrei.elkin@mariadb.com>