MDEV-3917 multiple use locks (GET_LOCK) in one connection.

The patch contributed by Konstantin Osipov applied.
    Native comments:
      Implement multiple user-level locks per connection.

      GET_LOCK() function in MySQL allows a connection  to hold at most
      one user level lock. Taking a new lock automatically releases the
      old lock, if any.

      The limit of one lock per session existed since  early versions
      of MySQL didn't have a deadlock detector for SQL locks.
      MDL patches in MySQL 5.5 added a deadlock detector,
      so starting from 5.5 it became possible to take multiple locks
      in any order -- a deadlock, should it occur, would be detected
      and an error returned to the client which closed the wait chain.

      This is exactly what is done in this patch: ULLs are moved
      to use MDL subsystem.

sql/sql_class.h

@@ -57,7 +57,6 @@ class Lex_input_stream;
 class Parser_state;
 class Rows_log_event;
 class Sroutine_hash_entry;
-class User_level_lock;
 class user_var_entry;
 
 enum enum_enable_or_disable { LEAVE_AS_IS, ENABLE, DISABLE };
@@ -1682,11 +1681,11 @@ public:
 
   HASH		handler_tables_hash;
   /*
-    One thread can hold up to one named user-level lock. This variable
-    points to a lock object if the lock is present. See item_func.cc and
+    A thread can hold named user-level locks. This variable
+    contains granted tickets if a lock is present. See item_func.cc and
     chapter 'Miscellaneous functions', for functions GET_LOCK, RELEASE_LOCK.
   */
-  User_level_lock *ull;
+  HASH ull_hash;
 #ifndef DBUG_OFF
   uint dbug_sentry; // watch out for memory corruption
 #endif