- Server incorrectly downgrading the MDL after prepare phase when
table is empty. mdl_exclusive_after_prepare is being set in
prepare phase only. But mdl_exclusive_after_prepare condition was
misplaced and checked before prepare phase by
commit d270525d and it is now
changed to check after prepare phase.

 - main.innodb_mysql_sync test case was changed to avoid locking
optimization when table is empty.