Convert fetch commands to async

The improvement depends on the number of recipes, network capabilities
and CPU power. This change is especially useful when there is nothing
to be actually downloaded because sources are up to date.

The --jobs/-j parameter allows to specify the number of parallel jobs to
run:
  - 2 jobs will be used if no -j parameter is specified
  - Twice the number of logical cores will be used if no number is
    provided after the -j parameter