While IDLE, the server sends unilateral responses to notify the client of
changes to the mailbox as they happen. Instead of always exiting the IDLE
on any change, allow the caller to pass a callback function which receives
the messages and returns an action to either Continue IDLE or Stop and exit.

For clients wishing to use the previous behaviour, a callback_stop convenience
function is provided that terminates the IDLE on any change to the mailbox.

Co-authored-by: Jon Gjengset <jon@thesquareplanet.com>

…Flags.

Also use it where we were previously iterating manually.

…e-responses

UnsolicitedResponse is not exhaustive, and open an issue if you find one
that isn't handled.