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.
xor encoding is fast enough for our purposes and provides
very good compression.
We remove all other ones that partially don't support floats
for the sake of simplicity.
