R1:0a6473138fd6

Purge readthrough caches before applying schema adjustments

Summary: Ref T1191. The bulk of the slowness in T1191 is copying tables. In some cases, we can't avoid this, but we have various readthrough caches which may be very large and are safe to drop, and dropping them is very quick (much less than 1 second). In particular, dropping the `changeset_parse_cache` made the process at least ~8 minutes faster on `secure.phabricator.com` (I killed it after 8 minutes, so I'm not sure what the real number is).