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.

This adds a basic compactor that will merge two persisted blocks into
one. It simply fully rewrites the index and concatenates the chunk
lists.

It just writes into the current working dir and doesn't properly handle
which blocks to compact for now.

This adds interval metadata to indexed chunks. The queried interval
is used to filter chunks when queried from the index to save
unnecessary accesses of the chunks file.

This is especially relevant for series that come and go often and larger
files.