network preconnect: make sure a request is using an in-flight pre-connect request

currently, a pre-connect request will only be used by another request when the pre-connect has finished (needs verification), because otherwise the pre-connect request will be marked as busy.
There needs to be a way to map normal requests to pre-connect requests, also to make sure not 2 normal requests are waiting for the same pre-connect request.
Also, what if an error occurs during the pre-connect request?