Add 3 cli commands: runner, listener and api-server
These commands do what they say, ie. start a runner, listener or API server
process.
Note that processes are not daemonized and run in front.
Typically used as:
swh-scheduler --cls local --database postgresql:///?service=swh api-server --host 127.0.0.1 --port 5008 swh-scheduler --cls remote --url http://127.0.0.1:5008 runner --period 10 swh-scheduler --cls remote --url http://127.0.0.1:5008 listener