This adds various new locks to replace the single big lock on
the head. All parts now must be COW as they may be held by clients
after initial retrieval.
Series by ID and hashes are now held in a stripe lock to reduce
contention and total holding time during GC. This should reduce
starvation of readers.
This changes the structure to a single WAL backed by a single head
block.
Parts of the head block can be compacted. This relieves us from any head
amangement and greatly simplifies any consistency and isolation concerns
by just having a single head.
NaN != NaN, so the previous code would incorrectly report
it as changed.
There's also plans to take advantage of the NaN payload,
so look at the entire value.
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.
