get rid of sessions, implement ClientIDs

[justinsheehy]

Remove the "session" concept from v2r2 and implement the ClientID concept as documented in the docs/ClientIDs.md document.

This not only more closely matches the VRR paper, it also should be cleaner and more understandable.

[andrewjstone]

This almost certainly needs to be implemented before real clients attempt to use the system. Otherwise client behavior will have to change with regard to idempotent retries (i.e. they won't have them if this isn't implemented.) Therefore I'm adding it to the diskless-log-kv milestone.