discovery: fix the base address of a relayed candidate (2)

The previous patch was broken since it prevented the unique priority
to be properly computed from the socket base address ip address. So
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.
12 jobs for foundation-of-relay-candidates in 14 minutes and 15 seconds
Status Job ID Name Coverage
  Build
passed #2586098
build autotools

00:00:54

passed #2586099
build meson

00:00:23

passed #2586101
docker windows 1809
build msvc amd64

00:05:43

passed #2586102
docker windows 1809
build msvc x86

00:04:27

passed #2586100
docker windows 1809
build msys2

00:12:51

 
  Test
passed #2586105
distcheck autotools

00:02:36

passed #2586108
doc-and-install meson

00:00:36

passed #2586103
test autotools

00:01:54

passed #2586104
test autotools valgrind

00:03:18

passed #2586954
test meson

00:01:22

passed #2586107
test valgrind meson

00:01:13

failed #2586106
test meson

00:01:02