Evaluate alternatives for YAJL 2

[mmd-osm]

When CGImap was started many years ago, YAJL was one of the options available at the time. Nowadays, more modern C++ based JSON libraries are available, such as https://github.com/nlohmann/json, which would also enable us to quickly add JSON based parsing. Maybe it's a good time to revisit YAJL.

[mmd-osm]

Closing this for now. I couldn't figure out how to do streaming like in yajl, without constructing the whole JSON object in memory.