Separate scheduler and process manager

[mxsasha]

The MirrorScheduler currently deals with running all periodic tasks. Initially this was import and export, now also RPKI and scope filter updates, which have been molded to look like mirror processes but are not quite the same. Next to scheduling, it also deals with process management.

This should be refactored, separating "what tasks should be running" from the management of the child processes themselves, and with a bit more abstraction to tasks in general, rather than mirroring only. Not a big refactor, but this will become unpleasant in the future otherwise. Maybe fix together with #578, depending on where that goes..