This patch fixes the problem described in LIBZMQ-205. The assertion itself
is probably caused by previously queued POLLIN events arriving after POLLIN
has been disabled on the socket.

The following additional bugs have been fixed as part of debugging this
problem:

- pgm_receiver_t does not flush messages written to the session in all
  cases which can lead to a stalled reader. Add calls to session->flush ()
  in the appropriate places.

- ensure to restart polling when a pending message is flushed in
  activate_in ().
Signed-off-by: Martin Lucina <martin@lucina.net>