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.
currently it's a flag, so really it's up to the user. We should give a good recommendation though.
@embark you're right about clock.MaxDrift() (or, after your rename, clock.MaxOffset()), it is also used in storage/{db,timestamp_cache{,_test}}.go and you want to change those as you go along :)