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.

channeldb/witness_cache_test.go

@@ -25,7 +25,7 @@ func TestWitnessCacheRetrieval(t *testing.T) {
 	witnessKey := sha256.Sum256(witness)
 
 	// First, we'll attempt to add the witness to the database.
-	err = wCache.AddWitness(Sha256HashWitness, witness)
+	err = wCache.AddWitnesses(Sha256HashWitness, witness)
 	if err != nil {
 		t.Fatalf("unable to add witness: %v", err)
 	}
@@ -59,13 +59,13 @@ func TestWitnessCacheDeletion(t *testing.T) {
 	// We'll start by adding two witnesses to the cache.
 	witness1 := rev[:]
 	witness1Key := sha256.Sum256(witness1)
-	if err := wCache.AddWitness(Sha256HashWitness, witness1); err != nil {
+	if err := wCache.AddWitnesses(Sha256HashWitness, witness1); err != nil {
 		t.Fatalf("unable to add witness: %v", err)
 	}
 
 	witness2 := key[:]
 	witness2Key := sha256.Sum256(witness2)
-	if err := wCache.AddWitness(Sha256HashWitness, witness2); err != nil {
+	if err := wCache.AddWitnesses(Sha256HashWitness, witness2); err != nil {
 		t.Fatalf("unable to add witness: %v", err)
 	}
 
@@ -107,7 +107,7 @@ func TestWitnessCacheUnknownWitness(t *testing.T) {
 
 	// We'll attempt to add a new, undefined witness type to the database.
 	// We should get an error.
-	err = wCache.AddWitness(234, key[:])
+	err = wCache.AddWitnesses(234, key[:])
 	if err != ErrUnknownWitnessType {
 		t.Fatalf("expected ErrUnknownWitnessType, got %v", err)
 	}