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-6eGjUCJORU3j now.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T8372

Differential Revision: https://secure.phabricator.com/D13095