Avoid deprecated IPv6 temporary addresses

[Rio Z]

Background: IPv6 Privacy Extensions (RFC 4941)-enabled host creates a
temporary address that is preferred for a short period of time
(usually 1 day), after which it becomes deprecated and a new temporary
address is generated. RFC 4941 §3.2 states that "deprecated address
can continue to be used for already established connections, but are
not used to initiate new connections."

RFC 4941 §6 discusses the problem of tracking deprecated addresses to
be removed. In particular, TCP connections can be easily tracked. But
for UDP-based applications like WireGuard, it's not straightforward as
it is usually connectionless.

Problem: Currently WireGuard keeps using the same temporary address to
send UDP packets to an existing peer even after the address becomes
deprecated. It should use the preferred (i.e. non-deprecated)
temporary address as soon as possible to satisfy the requirement of
privacy and also to make the deprecated addresses eligible to be
removed.