refactor: replace RecursiveMutex m_added_nodes_mutex with Mutex

The RecursiveMutex m_added_nodes_mutex is used at three places:
    - CConnman::GetAddedNodeInfo()
    - CConnman::AddNode()
    - CConnman::ThreadOpenConnections()
In each of the critical sections, only the the m_added_nodes member is
accessed (and in the last case, also m_addr_fetches), without any chance
that within one section another one is called. Hence, we can use an
ordinary Mutex instead of RecursiveMutex.

src/net.h

@@ -1100,7 +1100,7 @@ private:
     std::deque<std::string> m_addr_fetches GUARDED_BY(m_addr_fetches_mutex);
     Mutex m_addr_fetches_mutex;
     std::vector<std::string> m_added_nodes GUARDED_BY(m_added_nodes_mutex);
-    mutable RecursiveMutex m_added_nodes_mutex;
+    mutable Mutex m_added_nodes_mutex;
     std::vector<CNode*> m_nodes GUARDED_BY(m_nodes_mutex);
     std::list<CNode*> m_nodes_disconnected;
     mutable RecursiveMutex m_nodes_mutex;