JNI support for ReadOptions::iterate_upper_bound

JNI support for ReadOptions::iterate_upper_bound

Summary:
Plumbed ReadOptions::iterate_upper_bound through JNI.

Made the following design choices:
* Used Slice instead of AbstractSlice due to the anticipated usecase (key / key prefix). Can change this if anyone disagrees.
* Used Slice instead of raw byte[] which seemed cleaner but necessitated the package-private handle-based Slice constructor. Followed WriteBatch as an example.
* We need a copy constructor for ReadOptions, as we create one base ReadOptions for a particular usecase and clone -> change the iterate_upper_bound on each slice operation. Shallow copy seemed cleanest.
* Hold a reference to the upper bound slice on ReadOptions, in contrast to Snapshot.

Signed a Facebook CLA this morning.
Closes https://github.com/facebook/rocksdb/pull/2872

Differential Revision: D5824446

Pulled By: sagar0

fbshipit-source-id: 74fc51313a10a81ecd348625e2a50ca5b7766888