server/swap: avoid deadlock with maker redeem ack

When deleting a match after receiving the maker's redeem ack (of the
taker's redeem), do not violate established lock ordering of
Swapper.matchMtx and matchTracker.mtx.  This deals with the matchMtx
prior to locking the matchTracker, avoiding a possible deadlock with
another goroutine locking these.

server/swap/swap.go

@@ -1456,6 +1456,15 @@ func (s *Swapper) processAck(msg *msgjson.Message, acker *messageAcker) {
 	// actor.
 	mktMatch := db.MatchID(acker.match.Match)
 
+	// If this is the maker's (optional) redeem ack sig, we can stop tracking
+	// the match. Do it here to avoid lock order violation (a deadlock trap).
+	if acker.isMaker && !acker.isAudit { // getting Sigs.MakerRedeem
+		log.Debugf("Deleting completed match %v", mktMatch)
+		s.matchMtx.Lock() // before locking matchTracker.mtx
+		s.deleteMatch(acker.match)
+		s.matchMtx.Unlock()
+	}
+
 	acker.match.mtx.Lock()
 	defer acker.match.mtx.Unlock()
 
@@ -1486,10 +1495,6 @@ func (s *Swapper) processAck(msg *msgjson.Message, acker *messageAcker) {
 	if acker.isMaker { // maker acknowledging the redeem req we sent regarding the taker redeem
 		acker.match.Sigs.MakerRedeem = ack.Sig
 		// We don't save that pointless sig anymore; use it as a flag.
-		log.Debugf("Deleting completed match %v", mktMatch)
-		s.matchMtx.Lock()
-		s.deleteMatch(acker.match)
-		s.matchMtx.Unlock()
 	} else { // taker acknowledging the redeem req we sent regarding the maker redeem
 		acker.match.Sigs.TakerRedeem = ack.Sig
 		if err = s.storage.SaveRedeemAckSigB(mktMatch, ack.Sig); err != nil {