[net/refactor] Add connection type as a member var to CNode

- Directly maintaining the connection type prevents having to deduce it from
  several flags.

src/net.h

@@ -875,6 +875,7 @@ public:
 private:
     const NodeId id;
     const uint64_t nLocalHostNonce;
+    const ConnectionType m_conn_type;
 
     //! Services offered to this peer.
     //!