R1:074bf4ed7d54

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…