Encapsulate and centralize totalBufferUsed in ReplicationSourceManager

ReplicationSourceManager.totalBufferUsed is a counter, and is scoped to ReplicationSourceManager, but it is copied to ReplicationSource and ReplicationSourceWALReader, which makes the logic about ReplicationSourceManager.totalBufferUsed is scattered throughout ReplicationSourceManager,ReplicationSource,ReplicationSourceWALReader and ReplicationSourceShipper. It causes duplicated code and would make tracing the buffer usage somewhat difficult when there is problem about totalBufferUsed. I think we should encapsulate and centralize it in ReplicationSourceManager.