Ensure balena stays responsive whilst running blocking io operations

[pimterry]

Some context in https://github.com/resin-os/meta-resin/issues/1207 and https://app.frontapp.com/open/cnv_11ln7oz.

Some users want to enable the sync mount option, but this can make some balena commands extremely slow (10s of seconds). In the meantime, all other commands don't successfully complete, so the watchdog considering balena to have crashed. We should investigate making balena resilient to blocking i/o like this.

Front conversations

[splitice]

We have seen balena become unresponsive for long enough to trigger the watchdog when doing large amounts of IO on SD cards without the sync option.

So regardless of the status of sync improvements to reduce blocking IO would be a good idea.