Allow Mercator terrain to be handled in more than one thread

Currently all Mercator functionality is done in one background thread. This is a huge improvement from before when it was done in the main thread, but it would be even better if multiple threads could work on it at the same time.
The main issue here is when updating the terrain. What we need to do is to use shared locks for the Segments when reading data, and unique locks when altering it.
It might also be worth checking if we should supply our own Mercator::Terrain implementation, as the default one pretty much touches all Segments whenever something changes (such as an area being applied).