chain: use neutrino filters to speed up bitcoind seed recovery

In this commit, we use neutrino filters to speed up bitcoind seed
recovery. We use the recently created `maybeShouldFetchBlock` function
to check the filters to see if we need to fetch a block at all. This
saves us from fetching, decoding, then scanning the block contents if we
know nothing is present in them.

At this point, we can also further consolidate the `FilterBlocks`
methods between the two backends, as they're now identical.

chain/bitcoind_client.go

@@ -968,12 +968,38 @@ func (c *BitcoindClient) FilterBlocks(
 
 	blockFilterer := NewBlockFilterer(c.chainConn.cfg.ChainParams, req)
 
-	// Iterate over the requested blocks, fetching each from the rpc client.
-	// Each block will scanned using the reverse addresses indexes generated
-	// above, breaking out early if any addresses are found.
+	// Construct the watchlist using the addresses and outpoints contained
+	// in the filter blocks request.
+	watchList, err := buildFilterBlocksWatchList(req)
+	if err != nil {
+		return nil, err
+	}
+
+	// Iterate over the requested blocks, fetching each from the rpc
+	// client.  Each block will scanned using the reverse addresses indexes
+	// generated above, breaking out early if any addresses are found.
 	for i, block := range req.Blocks {
-		// TODO(conner): add prefetching, since we already know we'll be
-		// fetching *every* block
+		shouldFetchBlock, err := maybeShouldFetchBlock(
+			c.chainConn.client, block, watchList,
+		)
+		if err != nil {
+			return nil, err
+		}
+
+		// If the filter concluded that there're no matches in this
+		// block, then we don't need to fetch it, as there're no false
+		// negatives.
+		if !shouldFetchBlock {
+			log.Infof("Skipping block height=%d hash=%v, no "+
+				"filter match", block.Height, block.Hash)
+			continue
+		}
+
+		log.Infof("Fetching block height=%d hash=%v",
+			block.Height, block.Hash)
+
+		// TODO(conner): add prefetching, since we already know we'll
+		// be fetching *every* block
 		rawBlock, err := c.GetBlock(&block.Hash)
 		if err != nil {
 			return nil, err