[RocksJava] Support Snapshots

Summary:
Snapshots integration into RocksJava. Added support for the following functionalities:

- getSnapshot
- releaseSnapshot
- ReadOptions support to set a Snapshot
- ReadOptions support to retrieve Snapshot
- SnapshotTest

Test Plan:
make rocksdbjava
make jtest

Differential Revision: https://reviews.facebook.net/D24801
Integrated changes due to review bei ankgup87
Deprecate AtomicPointer

Summary: RocksDB already depends on C++11, so we might as well all the goodness that C++11 provides. This means that we don't need AtomicPointer anymore. The less things in port/, the easier it will be to port to other platforms.

Test Plan: make check + careful visual review verifying that NoBarried got memory_order_relaxed, while Acquire/Release methods got memory_order_acquire and memory_order_release

Reviewers: rven, yhchiang, ljin, sdong

Reviewed By: ljin

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D27543
Merge pull request #351 from fyrz/RocksJava_Snapshot_Support

[RocksJava] Support Snapshots
[RocksJava] - Hardening RocksIterator

RocksIterator will sometimes Sigsegv on dispose. Mainly thats related
to dispose order. If the related RocksDB instance is freed beforehand
RocksIterator.dispose() will fail.

Within this commit there is a major change to RocksIterator. RocksIterator
will hold a private reference to the RocksDB instance which created the
RocksIterator. So even if RocksDB is freed in the same GC cycle the
RocksIterator instances will be freed prior to related RocksDB instances.

Another aspect targets the dispose logic if the RocksDB is freed previously
and already gc`ed. On dispose of a RocksIterator the dispose logic will check
if the RocksDB instance points to an initialized DB. If not the dispose logic
will not perform any further action.

The crash can be reproduced by using the related test provided within this
commit.

Related information: This relates to @adamretter`s facebook rocksdb-dev group
post about SigSegv on RocksIterator.dispose().