COMPATIBILITY NOTE: scorch zap version bumped in this commit.
The version bump is because mergeFields() now computes whether fields
are the same across segments and it relies on the previous commit
where fieldID's are assigned in field name sorted order (albeit with
_id field always having fieldID of 0).
Potential future commits might rely on this info that "fields are the
same across segments" for more optimizations, etc.
The zap SegmentBase struct is a refactoring of the zap Segment into
the subset of fields that are needed for read-only ops, without any
persistence related info. This allows us to use zap's optimized data
encoding as scorch's in-memory segments.
The zap Segment struct now embeds a zap SegmentBase struct, and layers
on persistence. Both the zap Segment and zap SegmentBase implement
scorch's Segment interface.
inserted to the list of dv enabled fields list -
DocValueFields in mem segment.
Moved back to the original type `DocValueFields map[uint16]bool`
for easy look up to check whether the fieldID is
configured for dv storage.
docValues are persisted along with the index,
in a columnar fashion per field with variable
sized chunking for quick look up.
-naive chunk level caching is added per field
-data part inside a chunk is snappy compressed
-metaHeader inside the chunk index the dv values
inside the uncompressed data part
-all the fields are docValue persisted in this iteration
