compact L0 file to L0 when L0 is too small

in our case, we will do checkpoint by period, in some case, with period checkpoint, the L0 file is small, when L0 file number reach 4, it will trigger compaction and pick up L1 file together. which made huge write amplification。

in this case, maybe we can compaction L0 file to L0 instead of L1 ,when total size of L0 files is smaller than `target_file_size_base`