Factor gRPC orchestration modules out of uploader.py

[bileschi]

• Motivation for features / changes
  uploader.py contains many subclasses designed to organize the batching and timing of requests. The goal of this PR is to move them into their own individual modules, in preparation for adding asynchronous functionality to tensorboard uploader.

• Technical description of changes
  Moves the following independent classes into their own files under a new orchestration subdirectory. Adds some tests where in some cases unit testing was sparse.

  • BatchedRequestSender
  • BlobRequestSender
  • ByteBudgetManager
  • ScalarBatchedRequestSender
  • TensorBatchedRequestSender

• Detailed steps to verify changes work correctly (as executed by you)
  Beyond unit tests:

$ bazel run tensorboard:tensorboard -- dev upload --logdir ~/tensorboard-logdirs/demo/

and verified that the demo contains the right timeseries, graphs, etc.

• Alternate designs / implementations considered