TODO: Define Behavior for key, value

[akiozihao]

• [x] Clarify the behavior of key and value when they are nil or len=0 for different indexType.
• [x] Modify the test cases in hashtable_test to directly request the index instead of going through the DB.
• [ ] Add tests for different indexType in the DB, and test get in scenarios involving pendingWrites, activateMem, oldMem, and index.