Fix "Recursive nextTick" warning

Running a large number of concurrent requests causes a node warning and termination of the process in 0.10+

```shell
(node) warning: Recursive process.nextTick detected. This will break in the next version of node. Please use setImmediate for recursive deferral.
(node) warning: Recursive process.nextTick detected. This will break in the next version of node. Please use setImmediate for recursive deferral.
(node) warning: Recursive process.nextTick detected. This will break in the next version of node. Please use setImmediate for recursive deferral.
```

This change uses setImmediate, when available, in the proxy module.