Add a script for purging long-lived caches

Summary: See task; installs occasionally need to do this themselves, and a
script is much better than me telling them to truncate tables.

Test Plan:
Ran various flavors of this command:

• purge_cache.php
• purge_cache.php derp
• purge_cache.php --help
• purge_cache.php --all
• purge_cache.php --differential
• purge_cache.php --differential --maniphest

Then I verified the actual behavior:

• Visited a Differential revision with comments, observed cache update in

'Services' tab.

• Visited a Maniphest task with comments, observed cache update in 'Services'

tab.

• Reloaded a diff standalone view, got a cache update.

Reviewers: Makinde, btrahan, jungejason

Reviewed By: jungejason

CC: aran, jungejason

Maniphest Tasks: T676

Differential Revision: 1214