R1:e3f4f051fea2

Add --purge-user to bin/cache purge

Summary: Ref T4103. Provide a CLI mechanism for purging the user cache.

Test Plan:
- Purged with `--purge-user` and `--purge-all`.
- Verified cache table got wiped.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T4103

Differential Revision: https://secure.phabricator.com/D16033