pex: dial seeds when address book needs more addresses (#3603)
If we are low on addresses for peering, we need to discover more peers. The
previous behavior would query existing peers; however, if an existing peer
does not participate in peer exchange, then our node will not discover more peers.
This change consults both existing peers as well as seeds when there is a deficit
in address book addresses. This allows for discovering peers though existing channels
as well as via seeds if existing peers do not share addresses.