Singleton for requests session

Instead of instance-level, use a module level singleton so that in multiproc code,
subprocesses each have their own, reusable session.

Testing Done:
https://travis-ci.org/pantsbuild/pants/builds/39542444 passed

Bugs closed: 751

Reviewed at https://rbcommons.com/s/twitter/r/1264/
Sync of https://github.com/pantsbuild/pants/commit/98e395a7e8f234edf159c176ae3f3324efde30f9