llarp/context.cpp, llarp/nodedb.{h,c}pp: load netdb AFTER whitelist
llarp/router/router.cpp: explore always
llarp/router/{i,}rc_lookup_handler.{h,c}pp explore with whitelist, update routers with lookup before stale
This commit refactors functionality from the Router class into separate,
dedicated classes.
There are a few behavior changes that came as a result of discussion on
what the correct behavior should be.
In addition, many things Router was previously doing can now be provided
callback functions to alert the calling point when the asynchronous
action completes, successfully or otherwise.
