[JNI] Avoid a potential byte-array-copy btw c++ and java in RocksDB.get(byte[], byte[]).

Summary: Avoid a JNI call to GetByteArrayElements, which may introduce a byte-array-copy.

Test Plan: make jtest

Reviewers: haobo, sdong, dhruba

CC: leveldb

Differential Revision: https://reviews.facebook.net/D17451