implemented serveGet and clientGet

implemented serveGet and clientGet

Both are only at bare proof of concept stage. Still need to deal with
signaling validity and invalidity, and checking it.

And there's a bad bug: After -JN*2 requests, another request hangs!

So, I think it's failing to free up the Annex worker and end of request
lifetime.

Perhaps I need to use this:

https://docs.servant.dev/en/stable/cookbook/managed-resource/ManagedResource.html