routing/chainview: make filter updates synchronous for neutrino

This commit fixes a possible race condition wherein a call to
FilterBlock after a call to UpdateFilter would result in the call to
FilterBlock not yet using the updated filter. We fix this by ensuring
the internal chain filter is updated by the time the call to
FilterBlock returns.

routing/chainview/btcd.go

@@ -361,6 +361,7 @@ func (b *BtcdFilteredChainView) chainFilterer() {
 type filterUpdate struct {
 	newUtxos     []wire.OutPoint
 	updateHeight uint32
+	done         chan struct{}
 }
 
 // UpdateFilter updates the UTXO filter which is to be consulted when creating