Allows NM to signal to an agent that an in-progress secrets
request is no longer needed. This could happen if the device
for which the request was started was removed, disabled, or the
network being connected to changed, or whatever.
Filter registered agents for each secrets request to ensure that the
connection for which secrets are requested is visible to that agent,
and add that agent to the queue. Ask each agent in the queue until
one returns usable secrets. Ensure that if new agents register
or existing agents quit during the secrets request, that the queue
is updated accordingly, and ensure that an agent that's already
been asked for secrets, unregisters, and re-registers before the
secrets request is comple, isn't asked for secrets twice.
