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.

lnwallet/interface.go

@@ -274,9 +274,13 @@ type PreimageCache interface {
 	// argument. Otherwise, it'll return false.
 	LookupPreimage(hash []byte) ([]byte, bool)
 
-	// AddPreimage attempts to add a new preimage to the global cache. If
-	// successful a nil error will be returned.
-	AddPreimage(preimage []byte) error
+	// AddPreimages adds a batch of newly discovered preimages to the global
+	// cache, and also signals any subscribers of the newly discovered
+	// witness.
+	//
+	// NOTE: The backing slice of MUST NOT be modified, otherwise the
+	// subscribers may be notified of the incorrect preimages.
+	AddPreimages(preimages ...[]byte) error
 }
 
 // WalletDriver represents a "driver" for a particular concrete