chain: wait for bitcoind rpc to warm up

This modifies the `Start` method to handle the `RPC_IN_WARMUP` error
code of `-28` by retrying the rpc call to determine the current network
every second until it either succeeds or fails with a different code.
The current behavior fails and terminates the connection upon receiving
this error code. This change allows for connecting to a recently started
bitcoind node and starting the client while bitcoind is still warming
up.

Related issues: lightningnetwork/lnd#1533 &
https://github.com/ExchangeUnion/xud-docker/issues/195

chain/bitcoind_conn.go

@@ -147,9 +147,30 @@ func NewBitcoindConn(cfg *BitcoindConfig) (*BitcoindConn, error) {
 	}
 
 	// Verify that the node is running on the expected network.
-	net, err := getCurrentNet(client)
-	if err != nil {
-		return nil, err
+	var net wire.BitcoinNet
+
+	btcInWarmup := false
+	for {
+		net, err = getCurrentNet(client)
+		if err == nil {
+			// No error means bitcoind is responsive and we can proceed.
+			break
+		}
+		if rpcErr, ok := err.(*btcjson.RPCError); ok &&
+			rpcErr.Code == btcjson.ErrRPCInWarmup {
+			// Error code -28 indicates the bitcoind rpc is warming up.
+			// We will wait and recheck periodically until rpc is ready.
+			if !btcInWarmup {
+				btcInWarmup = true
+				log.Info("Waiting for bitcoind RPC to finish warming up...")
+			}
+			time.Sleep(time.Second * 1)
+		} else {
+			return nil, err
+		}
+	}
+	if btcInWarmup {
+		log.Info("Bitcoind finished warming up and RPC is ready")
 	}
 	if net != cfg.ChainParams.Net {
 		return nil, fmt.Errorf("expected network %v, got %v",