[STEP 1] Implement compaction mechanism

[kkli08]

• [ ] https://github.com/kkli08/KvDB/issues/17
• [ ] https://github.com/kkli08/KvDB/issues/34
• [ ] https://github.com/kkli08/KvDB/issues/19
• [ ] https://github.com/kkli08/KvDB/issues/20