Significantly improve zoom performance

[mourner]

With this pull in, zooming becomes significantly faster, especially noticeable when zooming out a lot. The code in datasource.js file was pretty inefficient. Here's the summary of changes:

• the code for retaining existing and caching tiles if they're children of a missing tile happened in a loop for no reason (you can see that z variable isn't used anywhere there), and it still didn't filter out tiles that have higher zoom that needed — this fix alone made a big difference
• sorting tiles from center out was slow because of Tile.fromID calls where they could be avoided
• tile ids where kept in arrays which was inefficient because of the need to keep the values unique (using lots of indexOf calls, utils.unique, etc.) — now it uses objects (and difference can also be calculated much faster)
• after all the fixes, I could reenable tile caching and pan tile without seeing any visual glitches, so I did

[ansis]

great

[kkaefer]

the code for retaining existing and caching tiles if they're children of a missing tile happened in a loop for no reason (you can see that z variable isn't used anywhere there), and it still didn't filter out tiles that have higher zoom that needed — this fix alone made a big difference

At one point, we gradually climbed up zoom levels to find the highest covering tile. Not sure where that got lost.

[mourner]

@kkaefer I thought so. It seems that to do this efficiently with the current code, the tiles need to be indexed by zoom.