Dont block executor when syncing files or mutexes

[ftilde]

I've been noticing for some time now that syncs sometimes take longer than expected (which is really confusing and annoying when you then accidentally execute old code on a remote machine...).

Investigation showed that the sync process itself is started late. The reason for that is that other tasks running subprocesses were doing so in a blocking fashion and thus preventing the executor from executing other tasks in the mean time. In this concrete example the mutex task was hindering a folder sync task from being executed. When syncing to two folders simultaneously the same could happen, though. Notably, this does not result in parallel sync to the same folder (which would be undesirable) because the folder sync task itself is waiting for the sync process to finish.

Fix: Use asyncio.subprocess instead of subprocess.check_output (or similar) and mark the functions async.

[falkoschindler]

Code looks good to me.
Assuming you successfully tested it, the PR is ready to merge. 👍🏻