core: do not GC units/jobs that are in the D-Bus queue

core: do not GC units/jobs that are in the D-Bus queue

Let's make sure that D-Bus messages are always sent out when pending,
before we might GC a unit/job.

This is kinda a follow-up for 8db998981a4fefd0122bcf5f965726b63c9045c2,
and a similar logic really applies: GC should only be done if we
processed everything else, generated evertyhing else and really don't
need it anymore.