Summary:
1. `WriteBufferManager` should have a reference alive in Java side through `Options`/`DBOptions` otherwise, if it's GC'ed at java side, native side can seg fault.
2. native method `setWriteBufferManager()` in `DBOptions.java` doesn't have it's jni method invocation in rocksdbjni which is added in this PR
3. `DBOptionsTest.java` is referencing object of `Options`. Instead it should be testing against `DBOptions`. Seems like a copy paste error.
4. Add a getter for WriteBufferManager.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4579
Differential Revision: D10561150
Pulled By: sagar0
fbshipit-source-id: 139a15c7f051a9f77b4200215b88267b48fbc487
6ecd26af27