Create shuffle service for external block storage

This task will be broken up into two parts – the first, being to refactor our internal shuffle service to use a BlockTransferService which we can easily extract out into its own service, and then the second is to actually do the extraction.

Here is the design document for the low-level service, nicknamed "Sluice", on top of which will be Spark's BlockTransferService API:
https://docs.google.com/document/d/1zKf3qloBu3dmv2AFyQTwEpumWRPUT5bcAUKB5PGNfx0