MXS-2067: Move server lock into Server class

The lock for the server is now only visible to the MaxScale core. Changing
the type to std::mutex also allows the use of RAII lock guards.

server/core/internal/server.hh

@@ -16,6 +16,10 @@
  * Internal header for the server type
  */
 
+#include <maxbase/ccdefs.hh>
+
+#include <mutex>
+
 #include <maxbase/average.hh>
 #include <maxscale/server.h>
 #include <maxscale/resultset.hh>
@@ -48,6 +52,9 @@ public:
         response_time->add(ave, num_samples);
     }
 
+    // TODO: Do all access to Server via methods
+    mutable std::mutex lock;
+
 private:
     // nantti, TODO. Decide whether to expose some of this in config, or if the values
     // can be calculated at runtime. The "500" or sample_max affects how often a