Instead of passing the PIDs back and forth in a handshake "Greet"
message, just use getsockopt(SO_PEERCRED) on both sides to get the same
information from the kernel.
This is a nice little simplification of the IPC protocol, although it
does not get rid of the handshake since we still have to pass the
"client ID" from the server to each client so they know how to refer
to themselves. This might not be necessary and we might be able to get
rid of this later on.
When draining the socket in IServerConnection, we would previously
handle each incoming (local endpoint) message as it came in.
This would cause unexpected things to happen while blocked waiting
for a synchronous response. That's definitely not what we want,
so this patch puts all of the incoming messages in a queue and does
a separate pass over the queue to handle everything in order.
This matches what we're already calling the server-side subclasses
better, though we'll probably want to find some better names for the
client-side classes eventually.
