Fix to support subscribing event from another thread
Bug: When itti_subscribe_event_fd() is called with new fd while the corresponding ITTI 'task' is blocked by itti_receive_msg(), the itti_receive_msg() never returns.
Scenario: In F1 and E1 split (MR !1629 (merged)), gtpv1Init() is called from outside of TASK_GTPV1.