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.
Just something to keep in mind is that engine.Store added a bunch of accessors to itself and might want to have a Context itself. Doesn't affect this part of the code though.