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.
This adds a basic compactor that will merge two persisted blocks into
one. It simply fully rewrites the index and concatenates the chunk
lists.
It just writes into the current working dir and doesn't properly handle
which blocks to compact for now.
Fabian Reinartz