server should allow setting a different port number that the one it is binding to

[psFried]

This is to allow for proxies. Say we use a proxy on port 5000 that fronts a flo server that binds on port 4000. That flo server needs to report to other servers in the cluster that it is listening on 5000. The only way for it to know this, of course, is if port 5000 is passed in as a separate command line argument (maybe --proxy-port?). This should just default to being the same as the value of the --port argument. When the server sends a PeerAnnounce or PeerUpdate message to another cluster member, it should set the port field on the message to the value of proxy_port instead of whatever port it is listening on.

This is probably going to be required in order to truly finish #4 since we'll likely need to standup some sort of proxy to simulate network issues.