Summary:
This is an abstraction for working with custom Comparators implemented in native C++ code from Java. Native code must directly extend `rocksdb::Comparator`. When the native code comparator is compiled into the RocksDB codebase, you can then create a Java Class, and JNI stub to wrap it.

Useful if the C++/JNI barrier overhead is too much for your applications comparator performance.

An example is provided in `java/rocksjni/native_comparator_wrapper_test.cc` and `java/src/main/java/org/rocksdb/NativeComparatorWrapperTest.java`.
Closes https://github.com/facebook/rocksdb/pull/3334

Differential Revision: D7172605

Pulled By: miasantreble

fbshipit-source-id: e24b7eb267a3bcb6afa214e0379a1d5e8a2ceabe