[JNI] Improve the internal interface between java and c++ for basic db operations.

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