* p2p/discover: remove ReadRandomNodes

Even though it's public, this method is not callable by code outside of
package p2p/discover because one can't get a valid instance of Table.

* p2p/discover: add Table.Nodes

* p2p/discover: make Table settings configurable

In unit tests and externally developed cmd/devp2p test runs, it can be
useful to tune the timer intervals used by Table.