Clean up and release resources used by the search view.
This will disconnect the aggregator cleanly from any resources it is using. It is recommended to call this method before finalising the search view, but calling it is not required.
Note that this will not unprepare the underlying aggregator: call IndividualAggregator.unprepare to do that. This allows multiple search views to use a single aggregator and unprepare at different times.
Concurrent calls to this function from different threads will block until preparation has completed. However, concurrent calls to this function from a single thread might not, i.e. the first call will block but subsequent calls might return before the first one. (Though they will be safe in every other respect.)
if unpreparing the backend-specific services failed — this will be a backend-specific error