Url can't be changed for easy reconnect

Using a client-side cursor (e.g. `lastId` processed); for a reconnect this wants to be passed back to the server in the query string (as cors doesn't allow custom headers and cors is always applicable to websockets).

However while `signalR.HubConnectionBuilder()` provides easy construction of the connection; the url cannot be changed after building.

This means if the connection disconnects and you need to pass a cursor (e.g. id); the connection has to be rebuilt and all the events re-registered to the new connection.

It would be easier if the url could be updated and only `.start` needed to be called again.