Repository indexers need to report how up to date their data is.

Currently, if a repository is in a queue to be updated (i.e the repository scanner is blocked else where) plugins have no way of finding out whether the data the FishEye is reporting is currently up to date. For plugins that store or cache data provided by FE/CRU, this means that the data they retain can easily be incorrect.

The repository indexing status reports if a repository is currently being indexed. We need a new feature that reports how current the data is, so that plugins caching or storing data will not cache or store invalid data.

Here is a complex usage to enhance the planning of this feature:

1. A ReviewContentUpdated event is emitted to a plugin.
2. The plugin waits before reindexing any new data, for the repository indexing status to report that it isn' currently indexing.
3. The plugin gets the data from FE/CRU and stores it.
4. The indexer resumes indexing.
5. plugin data is out of date - and the plugin can't tell without running tests on it self