Reduce the number of calls to ShuffleFile.getSize

[jealous]

ShuffleFile.getSize is a meta-data call of the file system. It could take several microseconds or even milliseconds in a shared file system. This would cause some overhead to the shuffle procedure.

When possible, we should use the recorded committed size or written bytes that are available in the memory instead of calling ShuffleFile.getSize to retrieve this information.