neutrino: implement BIP 151

[Roasbeef]

Currently neutrino is used primarily within lnd as an optional chain backend. Within the current LN protocol, all traffic is encrypted by default. However, our Bitcoin p2p traffic would still currently be in plaintext and unauthenticated. In order to make our traffic a bit more uniform, we should implement BIP 151 and switch to it for supported nodes. This is also important as before the filters are committed, our current security model relies heavily on a non-partitioning assumption, and several ISP level attacks can be mitigated with added encryption.

I'm tracking the issue here, but a bulk of the code will likely end up living in the peer package of btcsuite/btcd.