Closes#1581
This fixes the error in #1581, and also documents the purpose of this line. It ensures that if a peer tells us an address we know about, whose ID is the same as our current ID, we ignore it.
This removes the previous case where the ID's matched, but the IP's did not, which could yield a potential overwrite of the IP associated with the address later on. (This then would yield an eclipse attack)
This was not a vulnerability before though, thanks to a defensive check here 95fc7e58ee/p2p/pex/addrbook.go (L522))