The position mapper was intended to pre-computed "expensive" ordering
of label sets. It was expensive to update and caused a lot of trouble.
Skipping this optimization entirely actually revelead it was pointless
and even harmful from the e2e perspective.
This adds a position mapper that takes series from a head block
in the order they were appended and creates a mapping representing
them in order of their label sets.
Write-repair of the postings list would cause very expensive writing.
Hence, we keep them as they are and only apply the postition mapping
at the very end, after a postings list has been sufficienctly reduced
through intersections etc.
