Both _set_final_exception/result called for the same ResponseFuture

In case there are issues with connection to the server, and OperationTimedOut error is raised, ResponseFuture._errbacks are expectedly executed (in

method). However, the connection itself is not touched, and when it finally returns,

is called. As a result, we get both ResponseFuture._errbacks and ResponseFuture._callbacks executed.

Instead, driver code should make sure to have only one set of callbacks executed (i.e. if there is exception raised, don't execute ResponseFuture._callbacks).