Disable GET_LOCK()

Buggy code could contain GET_LOCK(). Since it is impossible (at least until Maria10.0.1 and My5.6) acquire more than 1 lock, STK/DBUG's locking locking mechanism would interfere with such locks. So, it will be possible to optionally disable GET_LOCK(). The locking will be made with:
* a worker_lock MEMORY table
* WHILE ... DO SLEEP(n)

Add option:
* locking_method ('get_lock', 'sleep')
in theory, more methods could be added if we find them or if MariaDB's future versions will give us better locking tools