Bug#13437900 - VALGRIND REPORTS A LEAK FOR REPL_IGNORE_SERVER_IDS

There was memory leak when running some tests on PB2.
The reason of the failure is an early return from change_master()
that was supposed to deallocate a dyn-array.

Fixed with relocating the dyn-array's destructor at ~LEX() that is
the end of the session, per Gleb's patch idea.
Two optimizations were done: the static buffer for the dyn-array to base on,
and the array initialization is called precisely when it's necessary rather than
per each CHANGE-MASTER as before.

sql/sql_repl.cc

@@ -1689,7 +1689,6 @@ err:
   thd_proc_info(thd, 0);
   if (ret == FALSE)
     my_ok(thd);
-  delete_dynamic(&lex_mi->repl_ignore_server_ids); //freeing of parser-time alloc
   DBUG_RETURN(ret);
 }