Improve how changing props is handled

[stevesims]

Changing props will cause new fetches to be initiated. If that happens whilst a fetch is already in progress things can get a little confusing, as the flags will get updated when the first fetch finishes, and thus will show that the fetch has completed when there's actually fetches in progress.

If props change multiple times whilst a fetch is already happening then the problem just gets a bit worse - plus the interim changes do not necessarily need to be seen.

[stevesims]

Some ideas for potential solutions to this.

1. a queue could be made of pending fetches. Only when the queue is empty should the isFetching flag be cleared
2. a flag could be set to indicate that a props have changed and data should be re-fetched, if there is currently a fetch in progress. This flag could be currentFetchStale

If possible we could/should abort a fetch before initiating a new one