Whether nothing uses this scope: it holds no resource and no scope or
span refers to it. ownRefs references are not counted; the resource
manager holds one on each peer scope it keeps.
Moves this scope under newEdges. Its current resources are counted in
the ancestors it gains and released from the ones it loses; an ancestor
it keeps (the system scope, usually) keeps counting them once. This is
how go-libp2p's SetPeer and SetProtocol juggle resources. Throws, and
changes nothing, if a gained ancestor's limit would be exceeded.