Pull orphaning + listing + adoption + deduplication

[stuartpb]

Need mechanisms in place so that:

• A listing can only have one pull going at a time
• If the frontend that initiated a pull disconnects, that pull is orphaned
• Another frontend can (re)connect to a listing's pull in progress
• A frontend can see what pulls are currently happening when opening

Basically, ports need to be abstracted away from pulls

[stuartpb]

It seems to me that the structure for this is a Map, by document ID per-pull-profile-listing, of objects containing arrays of ports listening to progress on a pull, as well as some sort of status (for newly-joining ports to be served).