Speed up response of /api/finish_channel

[aronasorman]

The implementation for that function (/views/internal.py:api_commit_channel) takes more than 5 minutes to reply for the KA chef. Let's optimize this to make it return a value faster by either parallelizing things or running it in a worker, and making the ricecooker poll an endpoint for progress.

[kollivier]

Leaving this open as we still want to speed up the process, even though it now completes.