Previously SNTXPCConnection had two-way validation which, due to the method of
implementation, forced a client to connect to a server straight away. Once that
was removed, it meant invalidationHandlers aren't called if either end dies
before the connection is established.
This also puts back the acceptedHandler, which can be used to know when the
connection has finished being established (particularly useful on the server
side), updates the __weak stuff to use WEAKIFY/STRONGIFY macros (and now
actually switch them back to strong within each block) and make the
tests a lot better by using in-process anonymousListener's rather than
lots of mocking.
Now that santad<->SantaGUI work more like the client/server they are,
having an SNTXPCConnection 'client' validate its server is no longer necessary.
Having the validation in the 'server' only simplifies the code.
Instead of having santad create a listener for SantaGUI to connect to
and then reverse the client-server relationship, have SantaGUI create an
anonymous listener that it sends to santad using the control interface.
Also add a queue for notifications so that blocks that occur while
SantaGUI isn't running will show up once it starts.
