refactor: Extract out common logic from Tensorboard BatchedRequestSender into a base class. (#592)

Exacted out common logic from the 3 BatchedRequestSenders into a base class. This will facilitate optimizations down the road. As nice side effects this brings up the uploader's test coverage due to fewer lines of code, and allows blob references to be batched instead of sent one at a time. unit test passed. Tried uploading all 3 types of data. Working well.

Author: yfang1