Fix memqueue getting stuck on shutdown

There was a case when openState.publish could get stuck and ignore its
shutdown signal, effectively preventing a Beat from gracefully
terminating.

This commit fixes this by ensuring every channel read/write also
checks for the shutdown signal.

libbeat/publisher/queue/memqueue/produce.go

@@ -143,7 +143,13 @@ func (st *openState) Close() {
 func (st *openState) publish(req pushRequest) (queue.EntryID, bool) {
 	select {
 	case st.events <- req:
-		return <-req.resp, true
+		select {
+		case resp := <-req.resp:
+			return resp, true
+		case <-st.done:
+			st.events = nil
+			return 0, false
+		}
 	case <-st.done:
 		st.events = nil
 		return 0, false