FTS - Indexes not getting deleted upon bucket deletion

Steps of reproduction

Create 10+ default indexes on bucket beer-sample.

Delete the bucket, 

Not all indexes are getting deleted.

This is due to a racy code in the DeleteAllIndexFromSource function and there looks bit inefficiency in the way we handle the index deletions there. Lots of repeated/redundant checks can be seen.

Along with the above simplification changes, making the whole target indexes set deletions atomic seems a win here.