raft: optimistically increase the next if the follower is already matched

Fix #1731 
/cc @yichengq @bdarnell

This is useful since we want to pipeline the appendEntry requests. Without
enabling optimistic increasing, the second pipelining appendEntry request
will include the entries the first one has already sent out. We decrease
the next directly to match if the leader receives a rejection for a matched
follower. This happens if one pipelining request get lost and following ones
arrives at the follower.
Does it matter if the follower has a match or not? Either the follower will accept this MsgApp, in which case optimistically increasing next is the right thing to do, or it will reject, in which case we need to try with a different value for pr.next instead of leaving it where it was.