discovery: prune local candidates with the same foundation

OpenVPN creates a tun interface having the same IP assigned twice like:

9: tun0: <POINTOPOINT,MULTICAST,NOARP,UP,LOWER_UP> mtu 1500 qdisc ...100
    link/none
    inet 10.222.111.10 peer 10.222.111.9/32 brd 10.222.111.10 scope global tun0
       valid_lft forever preferred_lft forever
    inet 10.222.111.10/32 brd 10.222.111.10 scope global tun0
       valid_lft forever preferred_lft forever

This interface would generate two identical host candidates with the same value
of foundation.
TEMP: Revert "agent: Avoid leaking UPnP mappings between streams"

This reverts commit 375cde8a652f8e367cf875bcb718a6d1abcfb254.

We don't have gupnp-igd 0.2.4 in the repository for this change to compile.