multi: make AddPreimage variadic, optimistically compute key

In this commit, we modify the WitnessCache's
AddPreimage method to accept a variadic number
of preimages. This enables callers to batch
preimage writes in performance critical areas
of the codebase, e.g. the htlcswitch.

Additionally, we lift the computation of the
witnesses' keys outside of the db transaction.
This saves us from having to do hashing inside
and blocking other callers, and limits extraneous
blocking at the call site.

witness_beacon.go

@@ -101,28 +101,41 @@ func (p *preimageBeacon) LookupPreimage(payHash []byte) ([]byte, bool) {
 	return preimage, true
 }
 
-// AddPreImage adds a newly discovered preimage to the global cache, and also
-// signals any subscribers of the newly discovered witness.
-func (p *preimageBeacon) AddPreimage(pre []byte) error {
-	p.Lock()
-	defer p.Unlock()
+// AddPreimages adds a batch of newly discovered preimages to the global cache,
+// and also signals any subscribers of the newly discovered witness.
+func (p *preimageBeacon) AddPreimages(preimages ...[]byte) error {
+	// Exit early if no preimages are presented.
+	if len(preimages) == 0 {
+		return nil
+	}
 
-	srvrLog.Infof("Adding preimage=%x to witness cache", pre[:])
+	// Copy the preimages to ensure the backing area can't be modified by
+	// the caller when delivering notifications.
+	preimageCopies := make([][]byte, 0, len(preimages))
+	for _, preimage := range preimages {
+		srvrLog.Infof("Adding preimage=%x to witness cache", preimage)
+		preimageCopies = append(preimageCopies, preimage)
+	}
 
 	// First, we'll add the witness to the decaying witness cache.
-	err := p.wCache.AddWitness(channeldb.Sha256HashWitness, pre)
+	err := p.wCache.AddWitnesses(channeldb.Sha256HashWitness, preimages...)
 	if err != nil {
 		return err
 	}
 
+	p.Lock()
+	defer p.Unlock()
+
 	// With the preimage added to our state, we'll now send a new
 	// notification to all subscribers.
 	for _, client := range p.subscribers {
 		go func(c *preimageSubscriber) {
-			select {
-			case c.updateChan <- pre:
-			case <-c.quit:
-				return
+			for _, preimage := range preimageCopies {
+				select {
+				case c.updateChan <- preimage:
+				case <-c.quit:
+					return
+				}
 			}
 		}(client)
 	}