Do not try to acquire structure_guard_mutex for the second time
when invalidating a table from send_result_to_client.


sql/sql_cache.cc:
  Do not try to acquire mutex when invalidating a table
  from send_result_to_client().
  A follow up patch for the patch for Bug#21074.
  Reuse code by moving locking-independent invalidation functionality
  into invalidate_table_internal.
sql/sql_cache.h:
  Add a new declaration.