chain: continue to accept []wire.OutPoint for rescan updates for bitcoind

In this commit, we fix a recently introduced bug that would cause
callers that attempted to get a spend notifications for a particular
outpoint to no longer get that precise notification. In a prior commit,
in preparation for the new neutrino gcs filter format, we added a
mapping from script to outpoint that will be used by neutrino, and later
other backends as well. However, we still need to match on outpoint
slices for bitcoind today.

chain/bitcoind.go

@@ -568,55 +568,65 @@ mainLoop:
 			// Update our monitored watchlists or do a rescan.
 			case event := <-c.rescanUpdate:
 				switch e := event.(type) {
+
+				// We're clearing the watchlists.
 				case struct{}:
-					// We're clearing the watchlists.
 					c.clientMtx.Lock()
 					c.watchAddrs = make(map[string]struct{})
 					c.watchTxIDs = make(map[chainhash.Hash]struct{})
 					c.watchOutPoints =
 						make(map[wire.OutPoint]struct{})
 					c.clientMtx.Unlock()
+
+				// We're updating monitored addresses.
 				case []btcutil.Address:
-					// We're updating monitored addresses.
 					c.clientMtx.Lock()
 					for _, addr := range e {
 						c.watchAddrs[addr.EncodeAddress()] =
 							struct{}{}
 					}
 					c.clientMtx.Unlock()
+
+				// We're updating monitored outpoints from
+				// pointers.
 				case []*wire.OutPoint:
-					// We're updating monitored outpoints
-					// from pointers.
 					c.clientMtx.Lock()
 					for _, op := range e {
 						c.watchOutPoints[*op] = struct{}{}
 					}
 					c.clientMtx.Unlock()
+				case []wire.OutPoint:
+					c.clientMtx.Lock()
+					for _, op := range e {
+						c.watchOutPoints[op] = struct{}{}
+					}
+					c.clientMtx.Unlock()
+
+				// We're updating monitored outpoints that map
+				// to the scripts that we should scan for.
 				case map[wire.OutPoint]btcutil.Address:
-					// We're updating monitored outpoints.
 					c.clientMtx.Lock()
 					for op := range e {
 						c.watchOutPoints[op] = struct{}{}
 					}
 					c.clientMtx.Unlock()
+
+				// We're adding monitored TXIDs.
 				case []*chainhash.Hash:
-					// We're adding monitored TXIDs from
-					// pointers.
 					c.clientMtx.Lock()
 					for _, txid := range e {
 						c.watchTxIDs[*txid] = struct{}{}
 					}
 					c.clientMtx.Unlock()
 				case []chainhash.Hash:
-					// We're adding monitored TXIDs.
 					c.clientMtx.Lock()
 					for _, txid := range e {
 						c.watchTxIDs[txid] = struct{}{}
 					}
 					c.clientMtx.Unlock()
+
+				// We're rescanning from the passed hash.
 				case *chainhash.Hash:
-					// We're rescanning from the passed
-					// hash.
 					err = c.rescan(e)
 					if err != nil {
 						log.Errorf("rescan failed: %s",