This patch implemented live data based GC for level merge.

Delete blob files which have (discardable size == file_size - file header - file footer) in OnCompactionCompleted()
Mark last two level blob files to merge in next compaction if discardable size reached GC threshold
Using live data based GC instead of regular GC as long as level merge is enabled.
Signed-off-by: Yi Wu <yiwu@pingcap.com>