[RocksJava] Flush functionality

Summary:
RocksDB now supports to flush Memtables.

Part of this pull request is the functionality located within
RocksDB (see flush methods) and the introduction of FlushOptions
as well as the backing C++ code.

Another aspect is the restructuring of the native class listing within
the Makefile. As it gets more and more unreadable its now one class
per line instead of all classes in one line.

Test Plan:
make rocksdbjava
make jtest

Reviewers: adamretter, yhchiang, ankgup87

Subscribers: dhruba

Differential Revision: https://reviews.facebook.net/D28593