Break AphrontWriteGuard dependency on AphrontRequest

Summary:
I want to move queryfx() and family to libphutil, for @chad and others (see T1283). We need to break a few dependencies to do this.

Since AphrontWriteGuard is independently useful, I broke the dependency between it and AphrontRequest rather than between Connection and WriteGuard. I'll move its implementation to libphutil in a future diff.

Test Plan: Loaded site, submitted CSRF form successfully, monkeyed with CSRF token, submitted CSRF form, got error.

Reviewers: btrahan, vrana

Reviewed By: vrana

CC: aran

Maniphest Tasks: T1283

Differential Revision: https://secure.phabricator.com/D3042