QGrpcHttp2Channel: fold the concrete-socket dispatch into a helper

The connect-timeout abort and the destructor flush duplicated the
socketType switch with its static_casts. Route both through a small
visitSocket() helper that invokes a callable with the socket downcast
to its concrete type.

Change-Id: Ie499f4fcdd0778d75fd295993fe717c7e1f5a9c0
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>