lnwallet+lnrpc: convert GetCurrentHeight to GetBestBlock in BlockChainIO

This commit expands the data returned by the current GetCurrentHeight
to also return the current best block hash, expanding the method into
GetBestBlock. Additionally, the current best BlockHash is also now
displayed within the GetInfo RPC call.

rpcserver.go

@@ -422,7 +422,7 @@ func (r *rpcServer) GetInfo(ctx context.Context,
 	pendingChannels := r.server.fundingMgr.NumPendingChannels()
 	idPub := r.server.identityPriv.PubKey().SerializeCompressed()
 
-	currentHeight, err := r.server.bio.GetCurrentHeight()
+	bestHash, bestHeight, err := r.server.bio.GetBestBlock()
 	if err != nil {
 		return nil, err
 	}
@@ -437,7 +437,8 @@ func (r *rpcServer) GetInfo(ctx context.Context,
 		NumPendingChannels: pendingChannels,
 		NumActiveChannels:  activeChannels,
 		NumPeers:           uint32(len(serverPeers)),
-		BlockHeight:        uint32(currentHeight),
+		BlockHeight:        uint32(bestHeight),
+		BlockHash:          bestHash.String(),
 		SyncedToChain:      isSynced,
 		Testnet:            activeNetParams.Params == &chaincfg.TestNet3Params,
 	}, nil