Let Primary and Secondary construct http.ServeMux

Deletes the fields {Primary,Secondary}.{Public,Internal}HTTPMux, and deletes the methods returning handler lists. Replaced with methods to create and return a http.ServeMux to handle the node's endpoints. Main functions just attach these muxes to the servers they create.

Also adds the prometheus /metrics to the internal mux, since that seems to be the intention. It was previously dangling.