Background worker to automate on-disk data housekeeping

This is an umbrella issue for the related projects.
There are number of cases[1] described in the issue attached that calls for a background worker entity that will do a housekeeping for:

1. empty values vacuum cleaning
2. on-disk persistant data invariants, e.g. tables clustering key ordering.
3. merging of multiple partitions made by separate bulk insertion operations.
4. on-disk persistant data values locality preservation to reduce extents ranges thus increasing the effect of Extent Elimination.

The worker entity must take concurrency control into account. As of MCS 6.1.1 the concurrency control includes on-disk data logical blocks and Extent Map entries.

1. Vacuum cleaner is actual but others are future projects yet to be implemented