Use tx diff to reclaim unused records.

Instead of recording dynamic records for labels as deleted in the
transaction log, use the fact that we store both the state before the
transaction and the state after, to only require storing the used ones.
The set of unused records can be computed as the difference between the
set of records used before and the set of records used after.

In effect reverts (most of) 069edcfe7dcbb07f43599c91ec76c154f49b1934.
Merge pull request #1450 from thobe/2.0-delete-by-delta

Use tx diff to reclaim unused records.