Find existing IngressId for reconnecting ingresses

[DRiKE]

Currently, any reconnecting ingress (e.g. a BMP session) will get a new IngressId, resulting in new (mui,prefix) combos in the store. In other words, food for the OOM killer (as witnessed in the testbed), as we do not remove entries from lost/disconnected ingresses.

• figure out per connector component which fields to match on to determine whether a session is a reconnect
• figure out to what extent we want to make this configurable (for now)
• implement for BMP
• implement for BGP
• implement for MRT, though that might need some additional thinking (matching updates to bviews and what not)