Server sends responses from different endpoints when different global IPv6 addresses are configured

[chrysn]

When a server is configured with different IPv6 addresses, and a aioCoAP server is bound to :: (which is currently the only way to get both v4 and v6 support in a single context), responses will always originate from the v6 address that is preferred by the kernel for packages to its destination, regardless of the destination address of the request that elicited it.

As source addresses can't be set in DatagramTransport.sendto, this needs dedicated endpoints.

[chrysn]

a powerdns article suggests that there is an easier workaround than listening to addresses individually: the IPV6_PKTINFO parameter from rfc 3542, which can be used both with recvmsg and sendmsg, and should be somewhat portable. this might not remove all reasons to do #34, but should at least allow sticking to :: for server socket binding.