Rework addnode behaviour

* Use CNode::addeName to track whether a connection to a name is already open
  * A new connection to a previously-connected by-name addednode is only opened when
    the previous one closes (even if the name starts resolving to something else)
  * At most one connection is opened per addednode (even if the name resolves to multiple)
* Unify the code between ThreadOpenAddedNodeConnections and getaddednodeinfo
  * Information about open connections is always returned, and the dns argument becomes a dummy
  * An IP address and inbound/outbound is only reported for the (at most 1) open connection

src/net.h

@@ -818,4 +818,14 @@ public:
 /** Return a timestamp in the future (in microseconds) for exponentially distributed events. */
 int64_t PoissonNextSend(int64_t nNow, int average_interval_seconds);
 
+struct AddedNodeInfo
+{
+    std::string strAddedNode;
+    CService resolvedAddress;
+    bool fConnected;
+    bool fInbound;
+};
+
+std::vector<AddedNodeInfo> GetAddedNodeInfo();
+
 #endif // BITCOIN_NET_H