Extend React.PureComponent instead of React.Component

[lencioni]

I was doing some profiling and I noticed that when Waypoint updates, we
call this._handleScroll synchronously, which eventually calls functions
that force layout. Since this happens during render, the layout is
immediately invalidated and then triggered again once rendering is
completed.

We avoid this problem in componentDidMount by deferring this work
until the next tick.

I think we should consider making two changes to Waypoint to help
address this:

1. Extend React.PureComponent instead of React.Component. In the
   place I saw this problem, we were rendering a waypoint with pure
   props, so the update in this case could be completely avoided by
   using PureComponent.

   <Waypoint
     onEnter={this.onEnterWaypoint}
     scrollableAncestor={this.props.scrollableAncestor}
   />

2. Defer this._handleScroll in componentDidUpdate using
   onNextTick, like we do in componentDidMount. This will still
   require layout, but it will happen around the same time that the
   browser was going to do layout anyway, avoiding the thrash.

This commit implements the first of these two interventions.

Partly addresses #263

[jamesplease]

(Emphasized part is highlighted)

Would this be a breaking change, as all children components of a Waypoint must now also be pure?

[lencioni]

I don't think so, since passing JSX children to a PureComponent deoptimizes the PureComponent, since JSX is essentially a function call that returns a new instance, so it is never pure.

This might surface existing problems with apps that expect a change in context to be picked up by consumers of that context as a re-render but happen to re-render for some other reason, but that's really a bug in that app's code and not a problem with using pure component here.

[lencioni]

@jamesplease @trotzig Any more thoughts on this or do you think we are good to merge?

[trotzig]

Looks good to me.