Use a Session to enable connection pooling

Requests and urllib3 automatically reuse connections if using a Session, which otherwise has all the same methods as the main Requests API, making this a drop-in replacement: http://docs.python-requests.org/en/latest/user/advanced/

In my tests, this change cut 830 HEAD requests to Foursquare's internal artifact cache from 22s to 11s.

Testing Done:
Using 830 cache keys copied from a real build, this change cut the time to do HEAD requests for each from 22s to 11s.

https://travis-ci.org/pantsbuild/pants/builds/38515947

Reviewed at https://rbcommons.com/s/twitter/r/981/
Sync of https://github.com/pantsbuild/pants/commit/cc8931e7378293fe5f962239a2d2da55e6d8a0a6