Provide a local cache for PhabricatorEnv

Summary: We hit some env config fairly often (~200 calls on the home page) and it's not especially cheap right now. This saves about 10ms on home, which is ~8% of the page weight on my machine.

Test Plan: Clicked around, everything seemed fine. Unit tests.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D5512