Split the AsyncResponse stuff out into it's own file

Signed-off-by: Rick Taylor <rick@tropicalstormsoftware.com>
Add destructor to AsyncResponseDispatcher - calls the callback with an empty stream

Signed-off-by: Rick Taylor <rick@tropicalstormsoftware.com>
Add length() to CDRStream - finally!

Signed-off-by: Rick Taylor <rick@tropicalstormsoftware.com>