NetworkManager StopClient Deadlock if Transport.ClientDisconnect doesn't check if already disconnected

[miwarnec]

for future reference.
reproduce:

• kcp2k v1.0
• build = server only
• editor (2020.1) = client only
• editor presses stopclient in networkmanagerhud
• this happens:

            NetworkManager StopClient 
              KcpTransport.ClientDisconnect()
                KcpConnection.Disconnect
              Kcp OnClientDisconnected
                NetworkManager OnClientDisconnectInternal
                  NetworkManager OnClientDisconnect
                    NetworkManager StopClient

encountered it because kcptransport ClientDisconnect does not check if already disconnected, so it would call it over and over again.

note that kcp clientdisconnect sets clientconnect=null AFTER the call. but the deadlock is IN the call.

in other words we need to avoid the self recursion in networkmanager stopclient.

[miwarnec]

should be fixed in master.
we added checks to NetworkClient.OnTransportDisconnected so it's only called once

[miwarnec]

reopening this because the fix in kcp was wrong.
it caused another kcp issue: #3704
instead we need to fix this in mirror if it still happens

[miwarnec]

nvm this doesn't happen anymore.
if it ever occurs again:

• StopClient should call disconnect
• OnDisconnected should never stop client

this is similar to very old UNET bugs/behaviour that we fixed the same way:

• DoSomething does something
• OnDidSomething never ever should call DoSomething again