merging the doc value length and loc
slices into a single offset slice as that
is enough to compute the starting offset and
length of the the doc values data for a given
document inside a docValue chunk.
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
