refactor(server) cleanup snapshot
Merge pull request #190 from xiangli-cmu/snapshot_cleanup

refactor(server) cleanup snapshot
Add Peer.LastActivity().
Merge pull request #194 from benbjohnson/peer-latency

Add Peer.LastActivity()
fix(server.go) fix potential deadlock when removing nodes
When server entries removePeer, it is holding log write lock. peer.stopHeartbeat might also need to acquire log read lock to finish. We need to make peer.stopHeartbeat non-blocking to fix the deadlock. Also actually there is no need to wait for the peer go-routine to stop.
Merge pull request #195 from xiangli-cmu/fix_remove_deadlock

fix(server.go) fix potential deadlock when removing nodes