Move mesido from subprocess to Celery worker process once Casadi releases the GIL

[lfse-slafleur]

Split from #54

Blocked by casadi/casadi#2955

As a workaround we currently moved Mesido to a subprocess which is terminated whenever the job is cancelled. We should move mesido back to the main process to simplify the worker once this is possible.

Also enable the progress function again!: #77