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 removes the flag from the label value index and serializes it using
encbufs.
TODO: move CRC32 checksum into label value index hash table for
referntial integrity.