MXS-2486: Make SSL configurations immutable

Changes to SSL configurations is expected to be rare which allows them to
be made into immutable objects once created. This is an acceptable
compromise between performance and usability.

include/maxscale/ssl.hh

@@ -151,12 +151,17 @@ public:
 
     SSLProvider(std::unique_ptr<mxs::SSLContext> context);
 
+    // Current configuration
     const mxs::SSLConfig& config() const;
-    mxs::SSLContext*      context() const;
-    void                  set_context(std::unique_ptr<mxs::SSLContext> ssl);
+
+    // The context or nullptr if no context is set
+    mxs::SSLContext* context() const;
+
+    // Set the context, argument must not be null
+    void set_context(std::unique_ptr<mxs::SSLContext> ssl);
 
 private:
-    mxs::rworker_local<std::shared_ptr<mxs::SSLContext>> m_context;     /**< SSL context */
-    mxs::SSLConfig                                       m_config;      /**< SSL configuration */
+    std::unique_ptr<mxs::SSLContext> m_context;     /**< SSL context */
+    mxs::SSLConfig                   m_config;      /**< SSL configuration */
 };
 }