public class SweepReducer
The reducer of a sweep job.
This reducer merges the small mob files into bigger ones, and write visited
names of mob files to a sequence file which is used by the sweep job to delete
the unused mob files.
The key of the input is a file name, the value is a collection of KeyValue where
the KeyValue is the actual cell (its format is valueLength + fileName) in HBase.
In this reducer, we could know how many cells exist in HBase for a mob file.
If the existCellSize/mobFileSize < compactionRatio, this mob
file needs to be merged.