[Java] Add basic Java binding for rocksdb::Env.

Summary: Add basic Java binding for rocksdb::Env.

Test Plan:
make rocksdbjava
make jtest
cd java
./jdb_bench.sh --max_background_compactions=1
./jdb_bench.sh --max_background_compactions=10

Reviewers: sdong, ankgup87, haobo

Reviewed By: haobo

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D18903