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This function is called frequently, let's avoid allocating as much as possible.

Unfortunately, constructing a router is _not_ cheap. Cache it so we don't have
to re-compute it every time we get a query.

Except deprecated warn calls

Allows specifying peer filter functions on query and on adding peers to the routing table. This patch also includes some reasonable default functions for a public-only and private-only DHT.
Co-authored-by: Will Scott <will@cypherpunk.email>
Co-authored-by: Steven Allen <steven@stebalien.com>