IF qe queue a new PDU to an empty outqueue then the mt service thread
will still be stuck in poll() until it timesout or the socket becomes
readable.
Fix this by sending SIGUSR1 to the service thread when we queue a PDU
to an empty queue. This will break out of poll() and we can immediately
go and write to the socket.
Signed-off-by: Ronnie Sahlberg <ronniesahlberg@gmail.com>