Problem: Issuing DROP TEMPORARY TABLE when in XA_IDLE state, would
trigger an assert when running with --log-bin. Root cause was that a
binlog handler for the DROP TEMPORARY TABLE statement was added to the
transaction in XA_IDLE state, but its removal was "optimized" away since
the statement was not actually doing anything.

Solution: Since the XA-spec actually prohibits DROP [TEMPORARY] TABLE
in XA_IDLE the problem can be avoided by returning ER_XAER_RMFAIL in
this case.

Temporary tables are also dropped when a connection ends. This implicit
DROP is binlogged as an explicit DROP statement. The assert (on the master)
does not manifest itself in this case because the implicit DROP has a
completely different code path which does not add a binlog handler to the
transaction even if we are in XA_IDLE.

The explicit DROP added to the binlog is NOT rejected on the slave because:
- in row mode the XA statements are not replicated, so the slave is not
  in XA_IDLE
- in stmt and mixed mode a COMMIT is inserted before the explicit DROP
  so the slave is no longer in XA_IDLE

Note! This patch changes behavior. Statements which would previously succeed
in XA_IDLE now fail with ER_XAER_RMFAIL. However, when running with
--gtid-mode=ON, an explicit DROP continues to fail with
ER_GTID_UNSAFE_CREATE_DROP_TEMPORARY_TABLE_IN_TRANSACTION