Extend testing coverage for 'hidden index'.
1. Ensure the query plan cache will be cleared for the given collection on hiding or unhiding an index.
2. Ensure hidden indexes will not be not visible for use by index filters.
3. Allow users to determine whether an index is hidden via the $indexStats aggregation stage.
4. Leave index usage statistics intact when an index is hidden. They will remain static due to non-use while hidden. Once unhidden they will continue from where they left off on use.
5. Replicate hidden index state on collMod or createIndexes
6. Persist hidden index state in the index catalog (i.e. hidden index remains hidden after restart)
7. Ensure the 'hidden' flag can be found in currentOp and profiler.