Guarantee the existence of the Phabricator access log

Summary:
We have a fair number of conditionals on the existence of the access log. Instead, always build it and just don't write it if the user doesn't want a version on disk.

Also, formalize logged-in user PHID (avoids object existence juggling) in the access log and move microseconds-since-startup to PhabricatorStartup (simplifies index.php).

Depends on D5532. Fixes T2860. Ref T2870.

Test Plan: Disabled access log, verified XHProf writes occurred correctly.

Reviewers: btrahan, chad

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2860, T2870

Differential Revision: https://secure.phabricator.com/D5533