Use `setImmediate` instead of `process.nextTick`

`setImmediate` yields to the event loop after callback is called to
avoid the possibility of I/O starvation, which could happen with
`process.nextTick`.

I discovered this when working on `seneca-loadbalance-transport`, it was
causing 'Maximum call stack size exceeded' errors, related to the limit
of recursive `process.nextTick` calls.