TEMPORARY + HANDLER + LOCK + SP".

Server crashed when one: 
1) Opened HANDLER or acquired global read lock
2) Then locked one or several temporary tables with
   LOCK TABLES statement (but no base tables).
3) Then issued any statement causing commit (explicit 
   or implicit).
4) Issued statement which should have closed HANDLER
   or released global read lock.
   
The problem was that when entering LOCK TABLES mode in the
scenario described above we incorrectly set transactional
MDL sentinel to zero. As result during commit all metadata 
locks were released (including lock for open HANDLER or
global metadata shared lock). Indeed, attempt to release
metadata lock for the second time which happened during
HANLDER CLOSE or during release of GLR caused crash.

This patch fixes problem by changing MDL_context's
set_trans_sentinel() method to set sentinel to correct 
value (it should point to the most recent ticket).

mysql-test/include/handler.inc:
  Added test for bug #51136 "Crash in pthread_rwlock_rdlock on 
  TEMPORARY + HANDLER + LOCK + SP".
mysql-test/r/flush.result:
  Updated test results (see flush.test).
mysql-test/r/handler_innodb.result:
  Updated test results (see include/handler.inc).
mysql-test/r/handler_myisam.result:
  Updated test results (see include/handler.inc).
mysql-test/t/flush.test:
  Added additional coverage for bug #51136 "Crash in
  pthread_rwlock_rdlock on TEMPORARY + HANDLER + LOCK +
  SP".
sql/mdl.h:
  When setting new value of transactional sentinel use 
  pointer to the most recent ticket instead of value 
  returned by MDL_context::mdl_savepoint(). 
  This allows to handle correctly situation when the new 
  value of sentinel should be the same as its current value 
  (MDL_context::mdl_savepoint() returns NULL in this case).