use setImmediate instead of process.nextTick

setImmediate more closely resembles async operations such as db or filesystem calls.

Tested on Windows, node v. 0.10.10

![async-cache-test](https://f.cloud.github.com/assets/1538739/1733035/6e0d5e24-632d-11e3-96ae-867ca816520c.PNG)