Add numNodesKnown to llarp.admin.dumpstate RPC endpoint

[notlesh]

This adds the number of nodes in the nodedb to the llarp.admin.dumpstate endpoint.

It might be better / more consistent to add an ExtractStatus() method to nodedb, though there doesn't seem to be much else in nodedb we might want to know.

Also note that num_loaded() is going to acquire the nodedb's access mutex; would this be problematic?

[majestrate]

the access mutex acquisition should not be a problem