Bug? Nested lists break on initial sort when `distance` prop is set

Hi @clauderic,

First off, thanks for a superb library! My team and I are big fans. :)

I've encountered a situation that I _think_ is a bug. If you:
* Nest multiple `SortableContainer` lists,
* Set a `distance` prop on each,
* Click within a member of the _outer_ list, and
* Try to drag a member of the _inner_ list,

...the drag behavior will break: the outer list item will move instead. After that, sorting the inner list will work as expected.

**Here's a repro: https://jsfiddle.net/bxz1716v/**
* Click within "Section B"
* Try to drag a child within the section (drag fails)
* Try again (drag succeeds)

(In that repro, the only change from your nesting demo at https://jsfiddle.net/8tw72bgy/12/ is the addition of`distance={10}` props.)

[](https://gyazo.com/47810b4748278077342621cf49940fa4)

So far as I can tell, the issue involves the fact that:
* [The `handleStart` handler sets `this.manager.active` regardless of whether `distance` is set](https://github.com/clauderic/react-sortable-hoc/blob/master/src/SortableContainer/index.js#L143-L172)
* [The `handleEnd` handler only cancels (unsets `this.manager.active`) if distance is _not_ set](https://github.com/clauderic/react-sortable-hoc/blob/master/src/SortableContainer/index.js#L224-L226)

So, that initial click in "Section B" sets the _outer_ list to active (and doesn't unset it on `mouseup`). Then the initial (broken) drag gets as far as [`handleSortEnd`, which sets `this.manager.active` to null](https://github.com/clauderic/react-sortable-hoc/blob/master/src/SortableContainer/index.js#L418). After that, child dragging works.

Does this appear to be a bug to you as well, or am I overlooking something?

Thanks!

P.S. I noticed that https://github.com/clauderic/react-sortable-hoc/issues/130 involves very similar conditions, but a different outcome -- not sure what to make of that yet.