Only fetch feeds from server when they are updated

[MyForest]

At the moment we fetch all the feeds as soon as one is updated on the server.

This means that if you have a feed updating every 6 seconds then we'll fetch the feed history (for the selected time window) for every feed every six seconds.

We should only refresh a feed's history if the feed history on the client is out of date.

[MyForest]

It's really tempting to add our newly-discovered "latest value" to the end of the list on the client. However we might get gaps in the data by doing that because our page may have been asleep for a while.

Notably this happens a lot on mobile where the page is put to sleep when it's not being viewed.

[MyForest]

Here we can see we're only refreshing the solar feed frequently because it was the only one that was updated.

[MyForest]

Doing it this way gives us the most adaptability to whatever update mechanism is being used for the underlying feeds. We'll have an up to date client without consuming a lot of bandwidth or wearing out the server.

[MyForest]

Ironically the most-updated feeds are the ones with the most data points. So, although this helps, it's not as amazing as we'd like it to be.