LoasdBalancer: the affinity keys are not used when completing the delayed (interim) transports

[zhangkun83]

InterimTransport {
  void closeWithRealTransports(Supplier<T> realTransports);
}

Although DelayedClientTransport saves the affinity keys for the delayed streams (in CallOptions), there is no way for it to pass those keys to Supplier<T>. This can be addressed by changing this method into:

TransportPicker<T> {
  T pickTransport(Attributes affinity);
}

InterimTransport {
  void closeWithTransportPicker(TransportPicker<T> picker);
}

Similarly:

DelayedClientTransport {
  void setTransportPicker(TransportPicker<T> picker);
}

[zhangkun83]

This is resolved in the LBv2 API (#2743)