FAB-16353 Don't waste memory in computing txid

The compute txid concatenates two byte slices and then computes the
hash.  The duplication of memory can be avoided by simply building the
hash iteratively.  This CR makes this small optimization.

Change-Id: Ibcd6d80e814099212b381b78432e1e4988ed6d41
Signed-off-by: Jason Yellick <jyellick@us.ibm.com>

Author: Jason Yellick

protoutil/proputils.go

@@ -366,8 +366,10 @@ func createProposalFromCDS(chainID string, msg proto.Message, creator []byte, pr
 func ComputeTxID(nonce, creator []byte) string {
 	// TODO: Get the Hash function to be used from
 	// channel configuration
-	digest := sha256.Sum256(append(nonce, creator...))
-	return hex.EncodeToString(digest[:])
+	hasher := sha256.New()
+	hasher.Write(nonce)
+	hasher.Write(creator)
+	return hex.EncodeToString(hasher.Sum(nil))
 }
 
 // CheckTxID checks that txid is equal to the Hash computed