Update layers/tiles while displaying

[kkaefer]

We should support updating tiles/layers while they are being displayed. This can happen when the tile data expires while it is still being displayed. It's also a prerequisite for #2445.

Prerequisites:

• Complete Split up buffers #2103 so we can drop GL buffers of old tiles

Likely obstacles:

• Figure out how to safely swap out buffers
• Tile data lifetimes
• Should API work on a per-layer or a per-tile level? Maybe both?

[willwhite]

@kkaefer what is your latest thinking around how to decide when to refresh the map? Will this be triggered manually or could we keep track of all tiles that have an Expires header and force a refresh when that date is hit? This would let the server decide when/if data should be refreshed.

[kkaefer]

The idea of this feature is to respect the Expires/max-age headers the server sends. We already do that today, but we never update a tile after it is being displayed. That means, that in situations where we display tiles or maps for a very long period of time, like in visualizations, we will continue to show a tile despite it being outdated. So ultimately, when we display a tile, we will also set a timer that will trigger a refresh once the server sent expires header is done.

[willwhite]

Gotcha, will it do the same for max-age or will the auto-refresh behavior be limited to expires?

[kkaefer]

On the client side, we calculate an absolute date when receiving the data, Expires will be taken verbatim; max-age will be added to the current time. After that, we don't discriminate between those two headers.

[kkaefer]

Subsumed by #2617.