RocksDB objects needs to be disposed after usage

The RocksDB JNI interface RocksObject has a dispose() function which need to be explicitly triggered after it is not reused, otherwise GC may not be able to de-reference it off-heap and hence effective lead to memory leak. See: https://github.com/facebook/rocksdb/issues/752#issuecomment-146511412

We need to make sure all library-controlled RocksDB objects are disposed after usage, and also indicate users to close those out-of-control objects.

Note that RocksDB community is also going to replace the dispose API by extending AutoClosable in the future, so this ticket may need to be re-visited when upgrade RocksDB versions:

https://www.facebook.com/groups/rocksdb.dev/permalink/870848569680325/