Created by: jetfuel

Move some functions definitions from headers to cc files.

The approach is to track what tag is modified in the storage during training. Only process the modified tablets at the PersistToDisk.

This should cover the first step in https://github.com/PaddlePaddle/VisualDL/issues/284. We should also look into if we can only call persistToDisk once at the end of training. Protobuf always serialize all data to the filesystem and that's a huge waste.