Summary:
Added a new iterator property: `rocksdb.iterator.internal-key` to get the internal-key (converted to user key) at which the iterator stopped.
Closes https://github.com/facebook/rocksdb/pull/3525

Differential Revision: D7033694

Pulled By: sagar0

fbshipit-source-id: d51e6c00f5e9d766c6276ef79774b81c6c5216f8