Add test-before-evict discipline to addrman

Changes addrman to use the test-before-evict discipline in which an
address is to be evicted from the tried table is first tested and if
it is still online it is not evicted.

Adds tests to provide test coverage for this change.

This change was suggested as Countermeasure 3 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

@@ -1824,11 +1824,18 @@ void CConnman::ThreadOpenConnections(const std::vector<std::string> connect)
             }
         }
 
+        addrman.ResolveCollisions();
+
         int64_t nANow = GetAdjustedTime();
         int nTries = 0;
         while (!interruptNet)
         {
-            CAddrInfo addr = addrman.Select(fFeeler);
+            CAddrInfo addr = addrman.SelectTriedCollision();
+
+            // SelectTriedCollision returns an invalid address if it is empty.
+            if (!fFeeler || !addr.IsValid()) {
+                addr = addrman.Select(fFeeler);
+            }
 
             // if we selected an invalid address, restart
             if (!addr.IsValid() || setConnected.count(addr.GetGroup()) || IsLocal(addr))