media-session: break rescan -> failed link -> rescan loop

If link fails and triggers a rescan, policy-node may try it again on next rescan, leading to infinite loop. Give up if seeing the same nodes as on previous rescan.

XXX: This change in principle should be safe, but I'm not sure if it can be triggered in current codebase. When originally tested against 04a13471 the repro was (while true; pactl switch BT profile on/off; done) with pavucenter open.