This CR implements a bare minimum raft-based chain, which only
supports single node. It does not support following features:
- process config type message
- raft snapshot
- raft WAL
- reconfigure raft

This CR all pulls in a fake clock library for testing purpose.

Change-Id: I5e93ff9eeb524ea1afd0348e1180fb9f4535fb5c
Signed-off-by: Jay Guo <guojiannan1101@gmail.com>