public class MemStoreWrapper
The wrapper of a DefaultMemStore.
This wrapper is used in the sweep reducer to buffer and sort the cells written from
the invalid and small mob files.
It's flushed when it's full, the mob data are written to the mob files, and their file names
are written back to store files of HBase.
This memStore is used to sort the cells in mob files.
In a reducer of sweep tool, the mob files are grouped by the same prefix (start key and date),
in each group, the reducer iterates the files and read the cells to a new and bigger mob file.
The cells in the same mob file are ordered, but cells across mob files are not.
So we need this MemStoreWrapper to sort those cells come from different mob files before
flushing them to the disk, when the memStore is big enough it's flushed as a new mob file.