monc: let users specify a callback when they reopen their monitor session

monc: let users specify a callback when they reopen their monitor session

Then the callback is triggered when a new session is established, and the
daemon can do whatever it likes. There are no guarantees about how long it
might take to trigger, though. In particular we call the provided callback
while not holding our own lock in order to avoid deadlock. This could lead
to some funny ordering from the user's perspective if they call
reopen_session() again before getting the callback, but there's no way around
that, so they just have to use it appropriately.

Signed-off-by: Greg Farnum <greg@inktank.com>