Follower: do not crash if contacted by master with a smaller commit index
Follower: do not crash if contacted by master with a smaller commit index

The following sequence of events can legitimitaly lead to this scenario:

* peer A (initial leader) commits entry with log index N.
* peer A is partitioned before it can send the new commit index to any
  follower.
* peer B is elected leader with commit index N - 1.
* peer A is unpartitioned, and receives a heartbeat from B with a greater term,
  but smaller commit index.

This is fine; peer B will roll the commit index forward once it commits a new
entry.

tschottdorf/raft,Hoverbear/raft,frewsxcv/raft-rs,danburkert/raft,tschottdorf/raft,frewsxcv/raft-rs,danburkert/raft,Hoverbear/raft