DDL workload".

When a RENAME TABLE or LOCK TABLE ... WRITE statement which
mentioned the same table several times were aborted during 
the process of acquring metadata locks (due to deadlock 
which was discovered or because of KILL statement) server 
might have crashed.

When attempt to acquire all locks requested had failed we
went through the list of requests and released locks which
we have managed to acquire by that moment one by one. Since 
in the scenario described above list of requests contained 
duplicates this led to releasing the same ticket twice and 
a crash as result.

This patch solves the problem by employing different approach
to releasing locks in case of failure to acquire all locks
requested. 
Now we take a MDL savepoint before starting acquiring locks 
and simply rollback to it if things go bad.

mysql-test/r/lock_multi.result:
  Updated test results (see lock_multi.test).
mysql-test/t/lock_multi.test:
  Added test case for bug #51134 "Crash in MDL_lock::destroy
  on a concurrent DDL workload".
sql/mdl.cc:
  MDL_context::acquire_locks():
    When attempt to acquire all locks requested has failed do
    not go through the list of requests and release locks which
    we have managed to acquire one by one. 
    Since list of requests can contain duplicates such approach
    may lead to releasing the same ticket twice and a crash as
    result.
    Instead use the following approach - take a MDL savepoint
    before starting acquiring locks and simply rollback to it
    if things go bad.