Index Scrubbing: support cleanup when dangling index entries are expected

[jjezra]

On some databases, as it seems, dangling index entries are part of the normal flow. To support routine cleanup of these entries, we should add:

1. OnlineIndexScrubber API argument to start from scratch (to avoid continuation of previous sessions)
2. OnlineIndexScrubber API argument to limit the number of deletes per time unit (to avoid FDB flooding)

With the existing way to avoid warning, the index scrubbing can be used as a vacuum method to remove expected dangling index entries.