Refactor RendezvousServer and rename to MailboxServer

[felinira]

"In general" sending arbitrary messages through the Mailbox is a valid way to use wormhole (i.e. no Transit or Dilation connection will ever appear). For example, the "invite" flows of Magic Folder and Tahoe-LAFS both do this, exchanging some small JSON messages both ways before shutting down the mailbox.

Yeah that is a fair point. I'd maybe want to clean up the API before making them public again though, it's a good opportunity. I'll put it on the list for later.

Originally posted in #307