app_voicemail: Cleanup mailbox topic and cache

app_voicemail wasn't properly cleaning up the stasis cache or the
mwi topic pool when the module was unloaded or when a user was
deleted as a result of a reload.  This resulted in leaks in both
areas.

* app_voicemail now calls ast_delete_mwi_state_full when it frees
  a user structure and ast_delete_mwi_state_full in turn now calls
  the new stasis_topic_pool_delete_topic function to clear the topic
  from the pool.

Change-Id: Ide23144a4a810e7e0faad5a8e988d15947965df8

apps/app_voicemail.c

@@ -2020,6 +2020,10 @@ static void free_user(struct ast_vm_user *vmu)
 		return;
 	}
 
+	if (!ast_strlen_zero(vmu->mailbox)) {
+		ast_delete_mwi_state_full(vmu->mailbox, vmu->context, NULL);
+	}
+
 	ast_free(vmu->email);
 	vmu->email = NULL;
 	ast_free(vmu->emailbody);

main/app.c

@@ -3344,6 +3344,9 @@ int ast_delete_mwi_state_full(const char *mailbox, const char *context, struct a
 	if (clear_msg) {
 		stasis_publish(mailbox_specific_topic, clear_msg);
 	}
+
+	stasis_topic_pool_delete_topic(mwi_topic_pool, stasis_topic_name(mailbox_specific_topic));
+
 	ao2_cleanup(clear_msg);
 	return 0;
 }