Remove nextTick

Prefer `setImmediate` over `process.nextTick`- My testing shows that when large amounts of the [stubbed](https://github.com/segmentio/integration/blob/master/lib/proto.js#L65) out methods run that are backed by [tick](https://github.com/segmentio/integration/blob/master/lib/proto.js#L395-L400), they drown out all the I/O since `process.nextTick` runs before any I/O.