atomic write service state and checks files

[camerondavison]

fixes #1221

[shoenig]

I think you will want to also call Sync on the tmpfile and parent directory for this to actually be atomic

[camerondavison]

doing a sync would help make it durable, should not have anything to do with it being atomic. I would say doing a sync for these files (which are just to help with entropy) is unnecessary, my original intention is just to make it so there never exists a 0 byte file. Without the sync there will either be a full byte file, or no file.

[rboyer]

If your goal is to never have a zero-byte file between system crashes, you must call sync on the file before the rename. If you don't care about the crash/reboot scenario then you can omit the Sync call.

[camerondavison]

okay. then this helps prevent corrupt state for consul restarts, and consul crashes but not for machine level crashes. I'll defer to the consul devs if they would like to add the sync, or just remove corrupt state files on startup.