Add fsync support

Enabling fsync will decrease the throughput significantly if you use flush a lot.
This benchmark flushes after every 1000 put operation.

(Absolute values are useless but relative values are somewhat interesting)

Benchmark                          (fsync) (type)   Mode   Samples         Mean   Mean error    Units
c.s.s.s.FsyncBenchmark.testFsync     false   NONE  thrpt        20  3058982.800   161256.424    ops/s
c.s.s.s.FsyncBenchmark.testFsync     false SNAPPY  thrpt        20  3435100.631   181324.302    ops/s
c.s.s.s.FsyncBenchmark.testFsync      true   NONE  thrpt        20    42128.340     1487.794    ops/s
c.s.s.s.FsyncBenchmark.testFsync      true SNAPPY  thrpt        20    46248.368     1311.363    ops/s