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.

htlcswitch/link.go

@@ -1417,7 +1417,7 @@ func (l *channelLink) handleUpstreamMsg(msg lnwire.Message) {
 		// any contested contracts watched by any on-chain arbitrators
 		// can now sweep this HTLC on-chain.
 		go func() {
-			err := l.cfg.PreimageCache.AddPreimage(pre[:])
+			err := l.cfg.PreimageCache.AddPreimages(pre[:])
 			if err != nil {
 				l.errorf("unable to add preimage=%x to "+
 					"cache", pre[:])

htlcswitch/mock.go

@@ -46,11 +46,13 @@ func (m *mockPreimageCache) LookupPreimage(hash []byte) ([]byte, bool) {
 	return p, ok
 }
 
-func (m *mockPreimageCache) AddPreimage(preimage []byte) error {
+func (m *mockPreimageCache) AddPreimages(preimages ...[]byte) error {
 	m.Lock()
 	defer m.Unlock()
 
-	m.preimageMap[sha256.Sum256(preimage[:])] = preimage
+	for _, preimage := range preimages {
+		m.preimageMap[sha256.Sum256(preimage)] = preimage
+	}
 
 	return nil
 }