snet.Conn support for roaming use cases

[semcodech]

Binding the snet.UDPConn to an unspecified address is not possible (prevented by SCIONNetwork.OpenRaw and SCIONNetwork.NewCookedConn, would be supported by the underlying net.UDPConn.ListenUDP though.

• Is the reason for this restriction related to the packet reception?
• What is the recommended way of handling roaming use cases?
  Should the interface's IP addresses continuously be monitored, the connection closed and a new connection opened, or is there a better way to handle such a scenario?

[semcodech]

Closed and opened in main project scionproto#4712

[juagargi]

There is an attempt of a multihomed socket implemented here:
juagargi#53
This is a draft PR just to keep track of the reviews.
I am waiting for preliminary reviews and then deciding whether to keep it separate or merging it to scionproto in a different PR, if possible.