common/RefCountedObj: cleanup con/des

common/RefCountedObj: cleanup con/des

Also, don't allow children to set nref (to 0). This is the more significant
change as it required fixing various code to not do this:

    <reftype> ptr = new RefCountedObjectFoo(..., 0);

as a way to create a starting reference with nref==1. This is a pretty
bad code smell so I've converted all the code doing this to use the new
factory method which produces the reference safely:

    auto ptr = ceph::make_ref<RefCountedObjectFoo>(...);

libradosstriper was particularly egregious in its abuse of setting the starting
nref. :(

Signed-off-by: Patrick Donnelly <pdonnell@redhat.com>