follow list fetching test and related changes and fixes.

- make BatchedSubManyEose() use a single duplicate id index and use it for replaceable loaders;
- fixes parsing follow entry from kind:3 events (and others);
- adds a "cause" to most cancelation errors in relay/pool;
- remove the inherent cache from dataloader (we have our own hopefully);
- increase max frame size we can read from any websocket to 2**18 (262k), which gives over 2000 item lists.

connection.go

@@ -20,6 +20,8 @@ func NewConnection(ctx context.Context, url string, requestHeader http.Header, t
 		return nil, err
 	}
 
+	c.SetReadLimit(262144) // this should be enough for contact lists of over 2000 people
+
 	return &Connection{
 		conn: c,
 	}, nil