m_tickets.front() == m_trans_sentinel'".

Debug build of server crashed due to assert failure in MDL
subsystem when one tried to execute multi-table REPAIR or
OPTIMIZE in autocommit=0 mode.

The assert failure occured when multi-table REPAIR or OPTIMIZE
started processing of second table from its table list and
tried to acquire upgradable metadata lock on this table.
The cause of the assert failure were MDL locks left over from
processing of previous table. It turned out that in autocommit=0
mode close_thread_tables() which happens at the end of table
processing doesn't release metadata locks.

This fix solves problem by releasing locks explicitly using
MDL_context::release_trans_locks() call.

mysql-test/r/repair.result:
  Added test for bug #50784 "MDL: Assertion `m_tickets.is_empty() ||
  m_tickets.front() == m_trans_sentinel'".
mysql-test/t/repair.test:
  Added test for bug #50784 "MDL: Assertion `m_tickets.is_empty() ||
  m_tickets.front() == m_trans_sentinel'".
sql/sql_table.cc:
  Ensure that metadata locks are released after multi-table REPAIR/
  OPTIMIZE/... processes each table. close_thread_tables() which is
  called after processing each table doesn't release metadata locks
  if we are in autocommit=0 mode. So this have to be done
  explicitly by calling MDL_context::release_trans_locks().