Summary:
Currently, `ReactUpdates` updates dirty components in increasing order of mount depth. However, mount depth is only relative to the component passed into `React.render`. This breaks down for components that invoke `React.render` as an implementation detail because the child components will be updated before the parent component.

This fixes the problem by using the order in which components are mounted (instead of their depth). The mount order transcends component trees (rooted at `React.render` calls).

Reviewers: @sebmarkbage @zpao

Test Plan:
Ran unit tests successfully:

```
npm run jest
```
Merge pull request #2570 from yungsters/updates

Update Dirty Components in Mount Ordering
Replace `mountDepth` with `isTopLevel`

Summary:
After #2570, `mountDepth` is only used to enforce that `setProps` and `replaceProps` is only invoked on the top-level component. This replaces `mountDepth` with a simpler `isTopLevel` boolean set by `ReactMount` which reduces the surface area of the internal API and removes the need to thread `mountDepth` throughout React core.

Reviewers: @sebmarkbage @zpao

Test Plan:
Ran unit tests successfully:

```
npm run jest
```
Merge pull request #2571 from yungsters/cleanup

Replace `mountDepth` with `isTopLevel`
Separate Child Reconciliation Step from Diffing

This separates the reconciliation step of children into a separate module.
This is the first step towards prerendering.

The stateful instances are reconciled and put into a "rendered children"
set. Updates creates a new of these sets. These two sets are then diffed
to create insert/move/remove operations on the set.

The next step is to move the ReactChildReconciler step to before the
native DOM component. That way it's possible to rely on child
reconciliation without relying on diffing.