Implement callback to check the state of the socket in fedmsg.tail_messages

[jmflinuxtx]

Currently, when using fedmsg.tail_messages, if the client falls off of the bus for some reason, there is no way of knowing. Messages get missed, but we don't know that there is anything wrong. It would be very handy to have a callback to verify the status so that a connection can be monitored and we can restart if no longer active.

[ralphbean]

FWIW, under the hood zeromq should try to repeatedly reconnect if the connection is dropped. We do however need a way to notify our caller that an outage has occurred.