discovery: use the address of a relayed candidate for its foundation

discovery: use the address of a relayed candidate for its foundation

According to the RFC, the base address of a relayed candidate is that
candidate itself. This precision is interesting and not just
anecdotical, since it allows the relayed candidates from the same turn
server to share the same foundation, and to not be tested concurrently.

However, since the base address in its current form is required to
compute a unique priority, we choose the keep the
candidate->base_address for this purpose, and to apply the RFC
consequence about relay candidate base being equal to the candidate
itself in the foundation assignment instead.