Fix memory leak problem caused by commit adcb636.

Fix memory leak problem caused by commit adcb636.

Commit adcb636 introduces "pending message queue". When a message
arrives, the info is added to the queue and a copy of object is
created at the same time, but forgoto free the object. Fix is,
creating a new function pool_pending_message_free_pending_message()
and call it after pool_pending_message_add(),
pool_pending_message_get() and pool_pending_message_pull_out().

Problem reported by Sergey Kim.