[Java] Add RocksObject, the base class of all java objects with a c++ pointer.

Summary:
Add RocksObject, the base class of all java objects which has a c++ pointer.
While the finalizer of a RocksObject will release its c++ resource, it is
suggested to call its RocksObject.dispose() to manually release its
c++ resource.

Existing RocksDB java classes are now extending RocksObject.

Test Plan:
make rocksdbjava
make jtest
make jdb_bench

Reviewers: haobo, dhruba, sdong, ankgup87, rsumbaly, swapnilghike, zzbennett

Reviewed By: haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D18411