Release nameplates automatically

[jml]

We provide a release function, but we don't really have a recommended way of calling it.

The protocol documentation says that "Nameplates (on the server) must live until the second client has learned about the associated mailbox, after which point they can be reused by other clients. " This implies that the client should release once it has confirmed knowledge of the second client.

In our code, this would be after we get the first readFromMailbox result that's from a side that's not us.