Always use a 50% chance to choose between tried and new entries

This change was suggested as Countermeasure 2 in
Eclipse Attacks on Bitcoin’s Peer-to-Peer Network, Ethan Heilman,
Alison Kendler, Aviv Zohar, Sharon Goldberg. ePrint Archive Report
2015/263. March 2015.

src/net.cpp

@@ -1221,8 +1221,7 @@ void ThreadOpenConnections()
         int nTries = 0;
         while (true)
         {
-            // use an nUnkBias between 10 (no outgoing connections) and 90 (8 outgoing connections)
-            CAddress addr = addrman.Select(10 + min(nOutbound,8)*10);
+            CAddress addr = addrman.Select();
 
             // if we selected an invalid address, restart
             if (!addr.IsValid() || setConnected.count(addr.GetGroup()) || IsLocal(addr))