R1:c42f767e3785

Destroy changeset parse cache when destroying a revision

Summary: When we delete a DifferentialChangeset, also delete the cache if it exists.

Test Plan:
Destroyed a revision, verified cache was destroyed. See marked line below.

$ ./scripts/differential/destroy_revision.php D1 --trace
>>> [0] <connect>
<<< [0] <connect> 1,324 us
>>> [1] <query> SELECT * FROM `differential_revision` WHERE `id` = 1
<<< [1] <query> 517 us

Really destroy 'D1: asdb' forever?