Change index persistence for series to not be accumulated in memory
before being written as one large batch. `Labels` and `ChunkMeta`
objects are reused.
This cuts down memory spikes during compaction of multiple blocks
significantly.
As part of the the Index{Reader,Writer} now have an explicit notion of
symbols and series must be inserted in order.
This has been a common source of hard to debug issues. Its a premature
and unbenchmarked optimization and semantically, we want ChunkMetas to
be references in all changed cases.
Fabian Reinartz
Goutham Veeramachaneni