manifest fsync

[Radim Kolar <hs...@filez.com>]

>> should be fine, the manifest is not rewritten that often
its rewritten after each sstable flush? databases should do fsync only 
in checkpoint.

Fsync scenario in WAFL is that hard checkpoint is done after predefined 
number of log segments. On checkpoint fsync everything and then write 
checkpoint marker to log, then remove checkpointed logs.