Implement fsync at right places to increase robustness

[giovannipizzi]

See comments and discussion in #15; see also e.g. https://blog.gocept.com/2013/07/15/reliable-file-updates-with-python/, or various Stack Overflow pages like this.

[giovannipizzi]

This might affect performance - moving to 'minimal working version'

[giovannipizzi]

Started to add the logic in 5082148. It's just for loose objects, we need to do it also for packs later. Also, I'm running CI tests to see if this works on Windows (probably it won't, and/or it's not needed there?)
Moving this to later milestone since this is enough to test the most important part of performance, and the rest is for increased robustness