Now that we have signed peer records in libp2p, we should use them for peer addresses in the DHT.

Need to take these to completion:

libp2p/go-libp2p#747.
#516.

Would be nice to couple these with dial-backoffs so we can probably dial signed records as we discover them without worrying about misdials on records with higher sequence numbers(as and when we discover them).

@petar @Stebalien Will we have the dial-backoffs work merged for 0.6 ?