Implicit committer mappings are potentially expensively computed multiple times on every request

CommitterUserMappingManager.getUserForCommitter() expensively calculates implicit mappings by calling userManager.getFishEyeUsers(). The result is cached, but this was still observed to account for 5.3% of rendering time of the Anon homepage on an AtlasEye demotion.

Options to fix include any combination of the follow:

1. On top of caching, implicit mappings should potentially be stored in the db.

2. calculate implicit mappings at index time and store them.

3. rewrite the implicit mapping searcher to use direct HQL to find a relevant user, rather than loading the entire user list.