Bump commit interval to 30s

[agners]

A higher file system commit interval can help to decrease the amount of
writes. In tests, a commit interval of higher than 30s seems not to help
much in practice. Settle with 30s for now.

Some tests about this change can be found here:
https://community.home-assistant.io/t/researching-i-o-write-improvements/456227

[agners]

It seems like big distributions like Ubuntu leave it as default. But those are also geared towards desktop type systems, so might not be a good indicator. ArchLinux has it as a "performance improvement" tip set to 60s in the example. Debian Wiki has a tip to set to 600s. ChromeOS synchronizes it with vm.dirty_expire_centisecs, and sets both to 600s (see files/00-sysctl.conf, there have been discussions to lower it, but it seems it stayed at 600s. It seems that the main reason is not to save writes, but to make sure fewer wake-ups are necessary and therefor maximize runtime when running on battery).

All in all, I expect this to create a bit more issues where files get corrupted (like the key.json corruption we are seeing, see #1706). But in the end those are software bugs which should be fixed (either the software needs to use a save write mechanism, or handle empty files properly).

[pvizeli]

Let's test the 30s. I'm more worry about docker layers