node: allow nextTick infinite recursion

Removing the depth counter while processing the nextTickQueue made it
possible to run out of memory if in an infinite recursive loop using
nextTick(). There was also an edge case where too many callbacks were
pushed onto the nextTickQueue, while not actually being recursive.

This is being done to prevent possible cryptic FATAL ERROR messages from
popping up, and issues being posted about them.
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.