It is my understanding that the objtrimstore stevedore API function is
only to be called once when the object is complete, which I believe is
also in line with the comment on ObjExtend() that "The final flag must
be set on the last call".

If this understanding of the API is correct, we did not adhere to it in
the fetch code when we made a copy of an existing stale object after a
304 response: There, we iterated over the stale object and did not set
the final flag just once when the object was complete, but rather after
each storage segment was copied.

This commit fixes this, adds some pedentry to the simple storage and
extends b00062.vtc to test this behavior specifically. On top, g6.vtc
also triggered without the fix but the duplicate trim detection in place.

This issue has originally surfaced in the SLASH/fellow storage where the
trimstore function implicitly asserted to only be called once.

Ref 115742b0
Ref https://gitlab.com/uplex/varnish/slash/-/issues/33