The peer addresses are specified with the GUC
gp_interconnect_proxy_addresses, it can be reloaded on SIGHUP, we used
to only care about the newly added ones, however it is also possible for
the user to modify them, or even remove some of them.

So now we add the logic to classify the addresses after parsing the GUC,
we can tell whether an address is added, removed, or modified.

The handling of the classified addresses will be done in the next
commit.