RavenDB-1391. Invalid usage of HMAC SHA1
Separate  deletion of primary index arifacts from the heavy storage, and delete the heavy storage in a background task
Merge branch 'New3' of github.com:ayende/ravendb into New3
Merge branch 'RavenDB-1399' of https://github.com/robashton/ravendb into New3
Register the delete index task in the pending tasks, to avoid stopping in the middle
- Properly register the index deletion in the pending tasks
- Restored deletion of indexes physically removed from index definitions
- Allow to cancel index deletion midway, which will stop, commit and resume on next db start.