Change signaling of support for the new `addrv2` messages to be done via
a dedicated message `sendaddrv2` instead of protocol bump.
The drawback of using a protocol bump is that the protocol version is
not a bitmask and if a node wants to announce support for `addrv2` this
would imply support for all prior features included in that protocol
version.
* Increase the maximum length of an address from 32 to 512 bytes and
clarify that the entire message should be rejected if it contains
a longer address.
(from https://github.com/bitcoin/bips/pull/766#issuecomment-519248699)
* Remove a contradiction about unknown address types - "MUST ignore" VS
"MAY store and gossip".
* Recommend gossiping addresses for networks to which the node is not
currently connected to.
(from https://github.com/bitcoin/bips/pull/766#issuecomment-545067608)
* Clarify that the entire message should be rejected if it contains an
address with unexpected size (e.g. IPv4 address with length 5).
