network/discovery: Add to DHT only peers that support genesis-based protocol (#3833)

This PR adds to the DHT only the peers that support the genesis/fork/kad
protocol.
Before this PR, any peer that supported the legacy `/kad/[id]` protocol
was added to the DHT.

This is the first step in removing the support for the legacy kad
protocols.

While I have adjusted unit tests to validate the appropriate behavior,
this still needs proper testing in our stack.

Part of https://github.com/paritytech/polkadot-sdk/issues/504.

cc @paritytech/networking

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Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>
Co-authored-by: Bastian Köcher <git@kchr.de>