raft: filter out messages from unknow sender.

If we cannot find the `m.from` from current peers in the raft and it is a response
message, we should filter it out or raft panics. We are not targetting to avoid
malicious peers.

It has to be done in the raft node layer syncchronously. Although we can check
it at the application layer asynchronously, but after the checking and before
the message going into raft, the raft state machine might make progress and
unfortunately remove the `m.from` peer.
Merge pull request #1871 from xiang90/fix_node

raft: filter out messages from unknown sender.