Reclaim space for removed rows in in-memory column sources

[rcaudy]

We should be more space efficient in cases like rolling-window row key spaces.

This can be done with an update committer that iterates rows in reclaimed (= current - previous) and current block-wise:

1. For the next row in reclaimed, find the impactedBlock. Advance to next block.
2. For impactedBlock, calculate the block start and end. Determine if there are any rows between start and end present in current, advance to after end.

This should let us determine blocks to remove, possibly more complicated in the sparse (multi-level) case.