[STEP 1] Implement the memtable as a balanced binary tree that supports put and get.

[kkli08]

• [ ] https://github.com/kkli08/KV-Store/issues/5
• [ ] https://github.com/kkli08/KvDB/issues/1
• [ ] https://github.com/kkli08/KV-Store/issues/2
• [ ] https://github.com/kkli08/KV-Store/issues/3