Add long running migrations #60
Description
there are migrations whitch can run while the actuall project is running.
so I propose with #59 that if it's type "background" or so, to spawn such migrations at the end of doing normal migrations and let them run in a go routine in the background while report back that migration is successfull.
this migrations MUST 1. allow to be cancled and restarted later ... 2. will only after full run commit it's migration task signature to the migrations task table ...