Force flush changes to filesystem

👋 Thank you for writing JFS!
I am using it as an alternative store for a side project.
While running my tests, I noticed that my `delete` case would sporadically fail.
It does a bunch of inserts, then a bunch of deletes and then reads to see if they still there.

I _think_ adding a `f.sync_all()?` at the end of `save_object_to_file` should at least alleviate the case.

I can open a PR if you want.