support Server Preferred Address

This makes sense once we've finished implementing connection migration (#234). See section 9.6.

We'll need to take special care of the disable_active_migration transport parameter. Section 5.3.2 specifies:

The disable_active_migration transport parameter does not prohibit connection migration after a client has acted on a preferred_address transport parameter.

From this, my interpretation is the following: If disable_active_migration is set, it tells the client to not migrate:

• for the entire lifetime of the connection if preferred_address is not set
• if preferred_address is set, the client must first migrate to that address, and then it is allowed to migrate