Previously we would decode state directly into our current ticket_handler.
This is problematic if for some reason we fail to decode, because we end
up with half new state and half old state.

We are probably already in bad shape if we get an update we can't decode,
but we may as well be tidy anyway.  Decode into new_* temporaries and
update the ticket_handler only on success.
Signed-off-by: Sage Weil <sage@newdream.net>