Fix a race condition when refresh is called closely to defineSlots

[patrick-mcdougle]

Hello!

This bug was a doozy to find, but I think this is the code required to fix it.

If there was a pending (un-resolved) load promise with defineSlot commands sitting waiting to be added to the gpt command queue and refresh was called, refresh would run too soon, calling refresh before the slots were defined. By linking the refresh to the loadPromise, we guarantee that slots that are sitting in the load queue are defined before refresh is called.

This is my first contribution to this library and there's not a formal contributing instructions so please let me know if there's anything else you need from me.

[patrick-mcdougle]

I made some diagrams to attempt to illustrate this, but async is really hard to visualize. Basically all of the defineSlots calls run synchronously after the googletag promise resolves (but queuing things onto the googletag cmd queue).

Before

A call to refresh could enqueue a refresh too soon

After

Refresh is forced to queue behind the most recent call to load

[jaanauati]

@patrick-mcdougle this makes sense!, i'll try to get this tested and merged by the end of the week. Thank you!

[jaanauati]

lgtm