XMOverlay performance

I made two improvements for managing EnergyGlob entities:

1. Do not delete all entities when syncing with API energyGlobGuids, only add new ones and delete the ones not in the current cell. This prevents unnecessary saving & faulting all EnergyGlob entities.
2. Render all XM in a single XMOverlayView with Core Graphics. This is a big improvement, because recreating 100's of UIViews on every refresh is really expensive.

Time permitting, I'll do the same (at least combine in one MKOverlayView) for links and fields. I don't think it can be easily done with portals, because they need to be clickable ("annotation").