Proposal: unsubscribe field

Currently there is a subscribe and a resolve field on resolvers.

I propose an unsubscribe field as well that can be used to release server resources.

There is currently no way to tell running processes that the subscription is no longer active to release the resources. An example is a memory heavy simulation system that is provisioned on subscribing and needs to be torn down when the client closes the connection.

```ts
Subscriptions: {
mySub: {
subscribe: () => AsyncIterator
resolve: payload => resolvedPayload
unsubscribe: () => void
}
}
```

An implementation would have the server create a stack of unsubscribe functions on the socket connection that it adds to on each subscription call. When the socket it closed, it would call each of those unsubscribe functions.

Start by locating the subscription resolver and socket-connection lifecycle code; the issue names no specific files or tests. Read how subscribe and resolve are wired, then determine the cleanup lifecycle for multiple active subscriptions. Done means closed connections release all resources through the proposed unsubscribe behavior, with coverage for that lifecycle.

Written by the indexing model from the issue text.