exchange: dispatch all code in 'serveronly' to the appropriate submodule All code related to exchange is now in either 'evolve.obsexchange' or 'evolve.obsdiscovery', and the 'serveronly' extension is using their 'exthelper' to set itself up. As a side effect, the 'serveronly' extensions now enable better exchange when pushing from that server too.

capabilities: properly sort capabilities after updating them Capabilities are usually sorted. This is especially useful to avoid silly error in test.