if_ovpn: allow peer lookup by vpn4/vpn6 address

Introduce two more RB_TREEs so that we can look up peers by their peer
id (already present) or vpn4 or vpn6 address.
This removes the last linear scan of the peer list.

Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D37605