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().