swim: do not rebind when new 'port' is 0

SWIM internally tries to avoid unnecessary close+socket+bind
calls on reconfiguration if a new URI is the same as an old one.
SWIM transport compares <IP, port> couples and if they are
equal, does nothing.

But if a port is 0, it is not a real port, but a sign to the
kernel to find any free port on the IP address. In such a case
SWIM transport after bind() retrieves and saves a real port.

When the same URI is specified again, the transport compares two
addresses: old <IP, auto found port>, new <IP, 0>, sees they are
'different', and rebinds. It is not necessary, obviously, because
the new URI covers the old one.

This commit avoids rebind, when new IP == old IP, and new port is
0.

Part of #3234