multi: replace usage of fastsha256 with crypto/sha256

This commit removes all instances of the fastsha256 library and
replaces it with the sha256 library in the standard library. This
change should see a number of performance improvements as the standard
library has highly optimized assembly instructions with use vectorized
instructions as the platform supports.

server.go

@@ -1,6 +1,7 @@
 package main
 
 import (
+	"crypto/sha256"
 	"encoding/hex"
 	"errors"
 	"fmt"
@@ -124,7 +125,7 @@ func newServer(listenAddrs []string, notifier chainntnfs.ChainNotifier,
 		// TODO(roasbeef): derive proper onion key based on rotation
 		// schedule
 		sphinx:      sphinx.NewRouter(privKey, activeNetParams.Params),
-		lightningID: fastsha256.Sum256(serializedPubKey),
+		lightningID: sha256.Sum256(serializedPubKey),
 
 		persistentConnReqs: make(map[string]*connmgr.ConnReq),