cache and merge in new annotations with already loaded data

[timwaters]

This PR makes it so when annotation data is requested, it keeps, merges in, and removes duplicates with any existing data instead of just overwriting with new data. This means that a user, if they panned back to an area of the map they have already visited, should not see a few ms of no data while data is being requested and should see the previous style.

[embeepea]

I think if we are going to do this, then we also need to have a way to remove annotations from memory that are outside the current view. Otherwise as the user pans around, memory usage could grow without bound, right?

[timwaters]

Yeah, memory would grow as they pan around. We'd have to figure out how many objects a normal browser could handle. Some older ones would struggle more.
I imagine it would be possible to resize the annotation array to a set size, so that it keeps the most recently pushed objects (assumption is that they are geographically close to each other) and that resizing would work wrt memory, garbage collect etc.

Another way could be to only update the id array for the map and leave the annotations to be replaced as it is. If the user clicks on a polygon and the annotations hadn't been loaded yet, they wouldn't see any detail, so maybe there could be a new check for new info and re-populate the side panel, or just show a loading indicator until? The id array could be much larger but still wouldn't be able to have everything in it, I guess. Perhaps a loading indicator or refresh mechanism could be used for both approaches?