multi: store peer flap rate on disk on best effort basis

Since we will use peer flap rate to determine how we rate limit, we
store this value on disk per peer per channel. This allows us to
restart with memory of our peers past behaviour, so we don't give badly
behaving peers have a fresh start on restart. Last flap timestamp is
stored with our flap count so that we can degrade this all time flap
count over time for peers that have not recently flapped.

chanfitness/chanevent.go

@@ -84,11 +84,17 @@ type peerLog struct {
 	channels map[wire.OutPoint]*channelInfo
 }
 
-// newPeerLog creates a log for a peer.
-func newPeerLog(clock clock.Clock) *peerLog {
+// newPeerLog creates a log for a peer, taking its historical flap count and
+// last flap time as parameters. These values may be zero/nil if we have no
+// record of historical flap count for the peer.
+func newPeerLog(clock clock.Clock, flapCount int,
+	lastFlap *time.Time) *peerLog {
+
 	return &peerLog{
-		clock:    clock,
-		channels: make(map[wire.OutPoint]*channelInfo),
+		clock:     clock,
+		flapCount: flapCount,
+		lastFlap:  lastFlap,
+		channels:  make(map[wire.OutPoint]*channelInfo),
 	}
 }