revert asynchronous devices mounting.

[utam0k]

I had previously tried to mount asynchronously, but this made our integration tests unstable.
I reverted it once, but it was still there, so I deleted it.

[utam0k]

@tsturzl You may not have read much of the code around here, but please give it a light review if you like.

[tsturzl]

Looks good. I'm curious what might have been causing the issues. Any ideas? I've noticed in the async work I'm doing that I have had to make sure I call sync on the writer. I wouldn't have thought I'd have to do this when writing to a virtual filesystem, so maybe there is something not so obvious happening?

[utam0k]

@tsturzl
When I ran it with docker in my local, it worked normally, but I could only find out that it failed in the integration test.
If you have any problems with cgroups asynchronous support, I should seriously look into it.