[RocksJava] Makefile restructured
[RocksJava] Flush functionality

RocksJava now supports also flush functionality of
RocksDB.
Merge pull request #393 from fyrz/RocksJava-Flush

[RocksJava] Memtable flush functionality
Fix NewFileNumber()

Summary: I mistakenly changed the behavior to ++next_file_number_ instead of next_file_number_++, as it should have been: https://github.com/facebook/rocksdb/blob/344edbb044ff5c08a43e4a6e9344c5c861552c0e/db/version_set.h#L539

Test Plan: none. not sure if this would break anything. It's just different behavior, so I'd rather not risk

Reviewers: ljin, rven, yhchiang, sdong

Reviewed By: sdong

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D28557