Fix type trait usage in cache
Fix type trait usage in cache

Summary:
Couldn't pass something that was `const` as `hashable`, for example a `const std::string&`.

Cache has a specialization in okui which this fix is really for, I'm just putting it here as well for consistency. That will be in another review.

Test Plan: Used in viewer

Reviewers: vmrob

Reviewed By: vmrob

Subscribers: #engineering

Differential Revision: https://phabricator.btlive.tv/D1676

bittorrent/scraps,bittorrent/scraps,bittorrent/scraps,bittorrent/scraps