node: fix removing AsyncListener in callback

context._asyncQueue shouldn't be exposed as asyncQueue, as it allows
modification of queues already attached to an event. Which is not
supposed to happend. Instead context._asyncQueue should be copied.
v8: unbreak freebsd build

Pending review of https://codereview.appspot.com/31500043/

Fixes #6576.
figure out how #define V(PN,TN) PN ## _.Dispose() works