Stash the context on static methods and use that by default

This makes the non-static method calls able to use the stashed
context and avoid needing to pass it every time after that.

Change-Id: I3c8d3916eab4b11f1802a68a9c3324541e003efd
Change RPC services to pass context to the serializer functions

This would be a change to Oslo, but I think that code is going to look
differently than it does now anyway. So, do this for the moment. This
will stab _context into any instance on the way into a method via args
or on the way out of the rpc proxy on result.

Change-Id: I1a68e8172aa40594110c601f259c4e294f94435c
Remove context usage for method args in start/stop path

Change-Id: Idc733af494edb9de0c7b2bb348b0ea7bca562ffb