[checksum] Improves performance of ChecksumBlob::length()

It isn't necessary to serialize the protobuf to compute the serialized
length, use ByteSizeLong() instead.

common/checksum/ChecksumBlob.cpp

@@ -98,8 +98,9 @@ std::string ChecksumBlob::serialize() const {
 }
 
 size_t ChecksumBlob::length() const {
-  // Inefficient as it requires re-doing the serialization. Check if we really need this method.
-  return serialize().length();
+  common::ChecksumBlob p_csb;
+  ChecksumBlobToProtobuf(*this, p_csb);
+  return p_csb.ByteSizeLong();
 }
 
 void ChecksumBlob::deserialize(const std::string &bytearray) {

common/checksum/ChecksumBlobTest.cpp

@@ -160,7 +160,9 @@ TEST_F(cta_ChecksumBlobTest, serialize_deserialize) {
   checksumBlob1.insert(MD5, "1234567890123456");       // 128 bits
   checksumBlob1.insert(SHA1, "12345678901234567890");  // 160 bits
 
+  auto len = checksumBlob1.length();
   auto bytearray = checksumBlob1.serialize();
+  ASSERT_EQ(len,bytearray.length());
 
   ChecksumBlob checksumBlob2;
   checksumBlob2.deserialize(bytearray);