Fix memory leak in the curb instrumentation

On long-running connection Curb use-cases like
Elasticsearch clients with Curb transport the `request` variable
points to the same Curb object not just for one request but for all.

Therefore, for each succesful request following the first one,
`original_callback` points to the previous on_failure callback set by newrelic.
So, an infinite chain of callbacks is created since newrelic's on_failure
callback maintains a reference to `original_callback`.

The changes force `original_callback` to point to the actual original callback on
subsequent requests instead of pointing to the newrelic's callback.
That way no reference is maintained for newrelic's callback of the
previous request & the memory leak is prevented by the GC.

lib/new_relic/agent/instrumentation/curb/instrumentation.rb

@@ -161,9 +161,14 @@ def install_completion_callback(request, wrapped_response, segment)
           # our on_failure callback hook.
           def install_failure_callback(request, wrapped_response, segment)
             return if request._nr_failure_instrumented
+
             original_callback = request.on_failure
+            nr_original_callback = original_callback&.instance_variable_get(:@__newrelic_original_callback)
+            original_callback = nr_original_callback || original_callback
+
             request._nr_original_on_failure = original_callback
-            request.on_failure do |failed_request, error|
+
+            newrelic_callback = Proc.new do |failed_request, error|
               begin
                 if segment
                   noticible_error = NewRelic::Agent::NoticibleError.new(error[0].name, error[-1])
@@ -175,6 +180,9 @@ def install_failure_callback(request, wrapped_response, segment)
               end
               request._nr_failure_instrumented = true
             end
+            newrelic_callback.instance_variable_set(:@__newrelic_original_callback, original_callback)
+
+            request.on_failure(&newrelic_callback)
           end
 
           # on_failure callbacks cannot be removed in the on_complete