WITHOUT ENDING TRANSACTION

If statements causing implicit commit are executed with an XA
transaction in ACTIVE state, "The command cannot be executed
when global transaction is in the ACTIVE state" error is 
reported. This error causes the implicit commit to fail (as it should).

The problem was that if the implicit commit at the start of a
statement execution failed, we also tried to commit at the end 
of the statement. This commit would fail for the same reason.
However for this commit we assumed that the transaction 
was either successfully committed or rolled back and therefore
released transactional metadata locks. 

This meant that the objects accessed earlier in the XA transaction
no longer were protected and could be e.g. dropped by other
connections. This again could lead to assertions, valgrind warnings
and various other problems when the XA transaction later tried
to commit.

This patch fixes the problem by aborting the statement if the
implicit commit at the beginning of the statement execution fails.
This means that the release of transactional locks at the end
of execution of statements causing implicit commit, will be skipped.