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.

Signed-off-by: NGoutham Veeramachaneni <cs14btech11014@iith.ac.in>

Signed-off-by: NGoutham Veeramachaneni <cs14btech11014@iith.ac.in>

Signed-off-by: NGoutham Veeramachaneni <cs14btech11014@iith.ac.in>

Very much a WIP
Signed-off-by: NGoutham Veeramachaneni <cs14btech11014@iith.ac.in>

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.

* To be more specific about the contents of the byte slice.

Closes fabxc/tsdb#18

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.

This adds write path support for segmented chunk data files.
Files of 512MB are pre-allocated and written to. If the file size
is exceeded, the next file is started. On completion, files
are truncated to their final size.

This commit simplifies a SeriesWriter into a ChunkWriter and detaches
it entirely from the notion of a series and the series index.