Before this change, writeRoaringWithLen() would leverage a reused
bytes.Buffer (#A) and invoke the roaring.WriteTo() API.
But, it turns out the roaring.WriteTo() API has a suboptimal
implementation, in that underneath-the-hood it converts the roaring
bitmap to a byte buffer (using roaring.ToBytes()), and then calls
Write(). But, that Write() turns out to be an additional memcpy into
the provided bytes.Buffer (#A).
By directly invoking roaring.ToBytes(), this change to
writeRoaringWithLen() avoids the extra memory allocation and memcpy.
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.
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
