Support multiple Overpass server simultaneously

[steinbro]

We're currently limited to forwarding all requests to a single (albeit configurable) Overpass server. We should have a pool acrosswhich to load balance, perhaps specifying the region of the world for which each can be used. I'm imagining a future where the world's data is distributed among a set of modest servers hosted by different individuals/organizations.

[steinbro]

I started this in a branch called multi_data_source, but now I'm thinking this would be cleaner as a separate load-balancing server sitting between the app and the Overscape servers(s). This would also transparently allow for a heterogeneous mix of e.g. PostGIS, Overpass, and even pre-generated static tile servers. The main concern is performance -- the naive approach (in which the intermediate server fetches and returns the content to the client) would add considerable overhead to already suboptimal response times.