Don't do too much work in the finalizer - in particular, there's no
need to send RemoveMatch() messages to the bus daemon since we're
going to sever the connection and the bus will garbage collect
anyway. In this case it crashed the process.

Also add a test case that checks that the appropriate GDestroyNotify
callbacks are called when unreffing a connection with either 1)
exported objects; 2) signal subscriptions or 3) filter functions
.. yes, ideally apps would unregister such callbacks before giving up
their ref but that's not how things work :-)

Signed-off-by: David Zeuthen <davidz@redhat.com>