Destroyed networks can't hope to beat champions

In particular, this fixes a bug where a non-validated destroyed
network can't get disconnected after it's replaced by another
non-validated network because CS thinks it should keep it in
case it validates and beats the replacement network.

Test: new test for this : CSDestroyedNetworkTests
Change-Id: I5d7e413624d6fca28b06484e5369cd17e4a599a4