Summary:
Improve the internal interface between java and c++ for basic db operations
by including the RocksDB native handle (i.e., c++ pointer of rocksdb::DB)
as a input parameter of the internal interface.
This improvement reduces one JNI call per db operation from c++.
Test Plan: make test
Reviewers: haobo, sdong, dhruba
Reviewed By: sdong
CC: leveldb
Differential Revision: https://reviews.facebook.net/D17445