Provide a way to abort an async indexing run

In some cases where a user is tweaking the indexing config it can happen that he saves the config mid way which triggers a long indexing run. Currently there is no easy way to abort such a run and only way to avoid wasting time in the long indexing cycle is to shut down the system.

For such cases it would be good to provide an "abort" operation as part of IndexStatsMBean which user can invoke to abort any run safely and cleanly