Don't finalize sent or dispatched messages while under the connection lock

Don't finalize sent or dispatched messages while under the connection lock

Finalizing a message can trigger callbacks; that's bad, if we have a
connection locked.

In particular, if a message is received by the "left side", passed to
the "right side" and sent (as in test/relay.c (see the diagram there)
or in dbus-daemon), then finalizing that message could result in the
live messages counter for the left side, and the outgoing messages counter
for the right side, both being decremented while under either side's
lock.

After a message is dispatched on the left side, finalizing it now drops
the lock temporarily, to avoid this problem.

After a message is sent on the right side, finalizing it is now deferred
until the right side unlocks, by moving it to a new queue of
"expired messages" which is automatically cleared every time we release
the lock.

The "live messages" counter for the "left" connection will now explicitly
take the left connection's lock before decrementing, to avoid
manipulating watches without a lock.

Reviewed-by: Colin Walters <walters@verbum.org>
Bug: https://bugs.freedesktop.org/show_bug.cgi?id=34393