handling of savepoints in stored routines.
Fixed ha_rollback_to_savepoint()/ha_savepoint()/ha_release_savepoint()
functions to properly handle savepoints inside of stored functions and
triggers.
Also now when we invoke stored function or trigger we create new savepoint
level. We destroy it at the end of function/trigger execution and return back
to old savepoint level.
We should not allow explicit or implicit transaction commits inside
of stored functions or triggers (so in autocommit mode we should not
do commits after execution of sub-statement).
Also since we don't support nested statement transactions in 5.0,
we shouldn't commit or rollback stmt transactions while we are inside
stored functions or triggers. This should be fixed in later (>=5.1)
releases.
