Fix memory leak, handle hash table size

Fix memory leak, handle hash table size

(1) Fix memory leak of atspi_accessible_get_reading_material.
(2) Remove defunct accessible from application hash table.
  In case of chromium-efl, it emits lots of "defunct" events without
  "RemoveAccessible", and this makes application hash table too big
  which could cause a Memory Alert on TV product.
(3) If AT-server does not emit "RemoveAccessible", then there is not
  a chance to remove accessible from the application hash table. And
  then the memory usage of AT-client grows bigger and bigger. So this
  patch is using newly defined value HASH_TABLE_SIZE_MAX.

Change-Id: If60d1fa1714d8bbf73b48cf16b1e1274f5a30cb9