Move AphrontWriteGuard to libphutil

Summary: Toward a queryfx() in libphutil and general separation of Aphront from Phabricator.

Test Plan: Loaded some pages which use write guard.

Reviewers: btrahan, vrana

Reviewed By: vrana

CC: aran

Maniphest Tasks: T1283

Differential Revision: https://secure.phabricator.com/D3051