R1:7a9174fa21b7

Digest resource cachekeys to maximum length

Summary:
Fixes T8372. We currently store the entire resource path in the cachekey; shorten it to a maximum of 64 characters.

The key size is 128 and T8372 had an example of a 112-byte key which could run over 128 with a slightly-long cache namespace.

Test Plan:
- Not 100% sure this is the issue, but it seems likely.
- Dumped the cachekey before and after the change.
- Saw values like `celerity:/res/1432859262T/phabricator/e2460e8f/core.pkg.css…