Resource starved CPU prevents ready from happening

**Describe the bug**

When starting to watch a directory, if there is another task happening concurrently on the OS, the `ready` event does not fire on MacOS. When the `ready` event doesn't fire, even though `add` events have fired for files, those files do not trigger `change` events when they are changed. I have reproduced this on Sierra as well as in testing in Travis CI.



**Versions (please complete the following information):**

 - Chokidar version 3.0.2

 - Node version 10.15.3 (tested in 12, 10, and 8 in Travis CI)

 - OS version: MacOS Sierra



**To Reproduce**

I created a gist with some minimal code: https://gist.github.com/catdad/d7f0bff7b47d0796f994174aa078719c



```bash

git clone https://gist.github.com/d7f0bff7b47d0796f994174aa078719c.git

cd d7f0bff7b47d0796f994174aa078719c

npm install



# run watcher alone

node debug.js

# see that files are added and ready is logged



# run watcher through spawn task with other tasks

node spawn.js

# see that files are added but ready is not logged

```



In the `spawn.js` file, if you comment out the second spawn, you can see that even when it is spawned in a child process without other tasks, `ready` is fired as expected.

Steps to reproduce the behavior.



**Expected behavior**

The `ready` event should always fire consistently.

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