Summary:
After we had a lot of failures with maven.org downloads, we
wanted an alternative location for downloading binary dependencies.
Hosting them through github would have been good in terms of
organizational and network dependencies, but that approach seems to be
awkward (fake releases, so would need a 'rocksdb-deps' repo) and
strangely complicated for Facebook policy on open source repositories.

This commit moves the downloads (that are not officially hosted by
others on github) from my personal rocksdb fork to an S3 bucket owned
by the Facebook RocksDB AWS account. Facebook employees can access
this through an internal tool, and we should be able to grant permission
to outside collaborators.

Assuming this works out, I will back-port to older branches to stabilize
their CI testing as well.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6526

Test Plan: CI

Differential Revision: D20430130

Pulled By: pdillinger

fbshipit-source-id: df52394a65e0a57942db3039bdaade8a4d520cb2