Break addnode out from the outbound connection limits.

Previously addnodes were in competition with outbound connections
 for access to the eight outbound slots.

One result of this is that frequently a node with several addnode
 configured peers would end up connected to none of them, because
 while the addnode loop was in its two minute sleep the automatic
 connection logic would fill any free slots with random peers.
 This is particularly unwelcome to users trying to maintain links
 to specific nodes for fast block relay or purposes.

Another result is that a group of nine or more nodes which are
 have addnode configured towards each other can become partitioned
 from the public network.

This commit introduces a new limit of eight connections just for
 addnode peers which is not subject to any of the other connection
 limitations (including maxconnections).

The choice of eight is sufficient so that under no condition would
 a user find themselves connected to fewer addnoded peers than
 previously.  It is also low enough that users who are confused
 about the significance of more connections and have gotten too
 copy-and-paste happy will not consume more than twice the slot
 usage of a typical user.

Any additional load on the network resulting from this will likely
 be offset by a reduction in users applying even more wasteful
 workaround for the prior behavior.

The retry delays are reduced to avoid nodes sitting around without
 their added peers up, but are still sufficient to prevent overly
 aggressive repeated connections.  The reduced delays also make
 the system much more responsive to the addnode RPC.

Ban-disconnects are also exempted for peers added via addnode since
 the outbound addnode logic ignores bans.  Previously it would ban
 an addnode then immediately reconnect to it.

A minor change was also made to CSemaphoreGrant so that it is
 possible to re-acquire via an object whos grant was moved.

src/net.h

@@ -58,8 +58,10 @@ static const unsigned int MAX_ADDR_TO_SEND = 1000;
 static const unsigned int MAX_PROTOCOL_MESSAGE_LENGTH = 4 * 1000 * 1000;
 /** Maximum length of strSubVer in `version` message */
 static const unsigned int MAX_SUBVERSION_LENGTH = 256;
-/** Maximum number of outgoing nodes */
+/** Maximum number of automatic outgoing nodes */
 static const int MAX_OUTBOUND_CONNECTIONS = 8;
+/** Maximum number of addnode outgoing nodes */
+static const int MAX_ADDNODE_CONNECTIONS = 8;
 /** -listen default */
 static const bool DEFAULT_LISTEN = true;
 /** -upnp default */
@@ -135,6 +137,7 @@ public:
         ServiceFlags nRelevantServices = NODE_NONE;
         int nMaxConnections = 0;
         int nMaxOutbound = 0;
+        int nMaxAddnode = 0;
         int nMaxFeeler = 0;
         int nBestHeight = 0;
         CClientUIInterface* uiInterface = nullptr;
@@ -151,7 +154,7 @@ public:
     bool BindListenPort(const CService &bindAddr, std::string& strError, bool fWhitelisted = false);
     bool GetNetworkActive() const { return fNetworkActive; };
     void SetNetworkActive(bool active);
-    bool OpenNetworkConnection(const CAddress& addrConnect, bool fCountFailure, CSemaphoreGrant *grantOutbound = NULL, const char *strDest = NULL, bool fOneShot = false, bool fFeeler = false);
+    bool OpenNetworkConnection(const CAddress& addrConnect, bool fCountFailure, CSemaphoreGrant *grantOutbound = NULL, const char *strDest = NULL, bool fOneShot = false, bool fFeeler = false, bool fAddnode = false);
     bool CheckIncomingNonce(uint64_t nonce);
 
     bool ForNode(NodeId id, std::function<bool(CNode* pnode)> func);
@@ -414,8 +417,10 @@ private:
     ServiceFlags nRelevantServices;
 
     CSemaphore *semOutbound;
+    CSemaphore *semAddnode;
     int nMaxConnections;
     int nMaxOutbound;
+    int nMaxAddnode;
     int nMaxFeeler;
     std::atomic<int> nBestHeight;
     CClientUIInterface* clientInterface;
@@ -529,6 +534,7 @@ public:
     int nVersion;
     std::string cleanSubVer;
     bool fInbound;
+    bool fAddnode;
     int nStartingHeight;
     uint64_t nSendBytes;
     mapMsgCmdSize mapSendBytesPerMsgCmd;
@@ -626,6 +632,7 @@ public:
     bool fWhitelisted; // This peer can bypass DoS banning.
     bool fFeeler; // If true this node is being used as a short lived feeler.
     bool fOneShot;
+    bool fAddnode;
     bool fClient;
     const bool fInbound;
     bool fSuccessfullyConnected;