core: make GC more aggressive

Since we should allow registering/unregistering transient units with the
same name in a tight-loop, we need to make the GC more aggressive, so
that dead units are cleaned up immediately instead of later.

hence, execute the GC sweep on every event loop iteration and clean up
units. This of course, means we need to be careful with adding units to
the GC queue, which we already are since we execute check_gc() of each
unit type already when adding something to the queue.