cache session threads and reuse them if possible

src/include/switch_core.h

@@ -690,6 +690,9 @@ SWITCH_DECLARE(switch_core_session_t *) switch_core_session_request_by_name(_In_
 */
 SWITCH_DECLARE(switch_status_t) switch_core_session_thread_launch(_In_ switch_core_session_t *session);
 
+
+SWITCH_DECLARE(switch_status_t) switch_core_session_thread_pool_launch(switch_core_session_t *session);
+
 /*! 
   \brief Retrieve a pointer to the channel object associated with a given session
   \param session the session to retrieve from