asyn: introduce fsspec.asyn.fsspec_loop

[isidentical]

Resolves #670

[martindurant]

I think this is a decent thing to include - anyone have a reason why this shouldn't happen or is somehow dangerous?

[martindurant]

I am waiting for a decision on fsspec/s3fs#495 (I asked for more feedback) and then I can do a new round of releases.

[isidentical]

@martindurant it would be also really nice if you could please take a look at the callbacks, and maybe include them in the next release (if you see them as fit). Being able to use get_file/put_file APIs directly without making UI worse (no progress bars for total bytes transferred) would be a huge win for certain filesystems (e.g sshfs).