Popover doesn't clean up immediately after unmount

[dgieselaar]

I've recently started seeing warnings in our APM tests that use the EUIPopover component:

  console.error ../node_modules/react-dom/cjs/react-dom.development.js:530
    Warning: Can't perform a React state update on an unmounted component. This is a no-op, but it indicates a memory leak in your application. To fix, cancel all subscriptions and asynchronous tasks in the componentWillUnmount method.
        in EuiPopover (created by TransactionActionMenu)
        in TransactionActionMenu

I know that React was upgraded recently (439708a6f9), this might be why we're just seeing them now. The warning only shows when multiple tests are ran successively (I'm honestly not sure why).

I've looked into the EUIPopover component and it looks like some of the timeouts are not cleared after unmounting, which leads to the aforementioned warning:

1. eui/src/components/popover/popover.tsx

   Lines 401 to 406 in a0b3b70

   	setTimeout(() => {
   	this.setState({ isOpenStable: true }, () => {
   	this.positionPopoverFixed();
   	this.updateFocus();
   	});
   	}, durationMatch + delayMatch);

2. eui/src/components/popover/popover.tsx

   Lines 377 to 381 in a0b3b70

   	window.requestAnimationFrame(() => {
   	this.setState({
   	isOpening: true,
   	});
   	});

It's relatively innocent, and I don't think it's a memory leak, but it would be nice to prevent the warning without having to await a timeout in our tests.