[Bug] peer: prevent last block height going backwards

[zquestz]

Fix from btcsuite/btcd#1606

This modifies the UpdateLastBlockHeight function to ensure the new
height is after the existing height before updating it in order to
prevent it from going backwards so it properly matches the intent of the
function which is to report the latest known block height for the peer.

Without this change, the value will properly start out at the latest
known block height reported by the peer during version negotiation,
however, it will be set to lower values when syncing from the peer due
to requesting old blocks and blindly updating the height.

It also adds a test to ensure proper functionality.

This is a backport of decred/dcrd#1747

I think this might be the issue we see when syncing in Neutrino and the sync progress jumps around!