Protect against data loss by OS crash

[beorn7]

Inspired by @matthiasr and in the spirit of #590 , I was thinking about sync'ing series files, too.

In general, that would just kill performance, but with the newly designed persistence strategy #589 , the number of series file operations is reduced. We could at least offer a flag to sync after each write to a series file. (And the order should be sync (optional) - close - rename, not rename - close, as it is now.) One idea of mine (as a third option for the flag): An adaptive strategy, sync as long as the persist queue is mostly empty. once a certain watermark is reached, switch to non-sync'd writes. (Similar to what we do already with checkpointing.)

Independent from the above, we could/should just sync everything upon shutdown (man 2 sync). Not sure if there is a platform indpendent way to do that with Go.

[beorn7]

This is implemented now. I couldn't find a good way for an equivalent of running the sync command from within Go. So I leave it out for now. In normal situations, everything will by synced anyway.

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