Refactor and test init_additional_STUN_servers.

* Make it accept any server sequence.
* Make it idempotent.
Test and refactor.

* Import modules, not names.
* Stricten port tests.
* Rename check_NAT_status to get_NAT_status.
* Test is_ipv6_address.
* Fix docstrings and test get_peer_url.
Merge pull request #841 from Renelvon/test_network_util

Mini refactor and test network util
If port arg is set then obey it don't ignore it
Missing CryptoPeerConnection method bug
Why check again? Redundant
Catch legacy message unpack exception
Merge pull request #873 from hoffmabc/decline-legacy-msgs

Hybrid network fixes