Added ability to measure clock offset from cluster

RPC servers and clients now both update a shared map of measured remote offsets.
Servers get this measurement from their attached clients. Servers
are given this ability because they don't always have their own client to the
remote node if that remote node has a client to them.

The server now passes a Context object to the rpc frameworks rather
than passing all 3 required fields individually.

Though the algorithm for measuring cluster offset is now implemented, the server does not yet actively measure cluster offset nor does it do anything with any offset measuring.
I added TODOs in the server and in this class, and I agree that we should return an error.

I'm not sure about the lower bound. More clocks reduce noise, but we also include errors in this calculation, so we should theoretically be reading even 1 or 2 clocks correctly. It's a question that might require more thought or experimental results.