consensus

When new leader performs failover check and finds out leftover
ConfChange, it should re-propose that AND do not accept new env
until ConfChange is applied.

This CR also moves type b config failover check from `serveRaft`
to `serveRequest` go routine. `serveRaft` now only takes care
of raft related logics, therefore making the code easier to
reason about.

Change-Id: I4e32cb155abff52e7d1c0fbe4c6f2aa5e5ef1605
Signed-off-by: Jay Guo <guojiannan1101@gmail.com>