This commit checks http.Request.Context().Err() to see if the context
has been canceled before writing an error code. It uses the non-standard
Nginx 499 error code for client disconnection.
This new session service has the ability to work independant of other systems
it relies on having its own store type which should allow us to be more flexible
then using the built in kv system.
I have included an in mem session store.
Lyon Hill
Ben Johnson
Alirie Gray