pvescheduler: reworking child pid tracking

pvescheduler: reworking child pid tracking

previously, systemd timers were responsible for running replication jobs.
those timers would not restart if the previous one is still running.

though trying again while it is running does no harm really, it spams
the log with errors about not being able to acquire the correct lock

to fix this, we rework the handling of child processes such that we only
start one per loop if there is currently none running. for that,
introduce the types of forks we do and allow one child process per type
(for now, we have 'jobs' and 'replication' as types)

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>