This updates the github driver to cache PRs by sha using cachetool's
LRUCache.
We make this change because we need to cache closed PRs so can't rely on
the action of closing a PR to remove the PR from the cache. Since we
don't have a good method of evicting entries we fall back to LRU with
a reasonable cache size (2k commits).
Change-Id: I5fb6c8b33f9eed221a8b84e537f02e7dccf2d2df