Defined log_state_change() interface in server_service.

The interface method can be used to notify the DBMS implementation
about state changes in well defined order. The call will be done
under server_state mutex protection.

include/wsrep/server_service.hpp

@@ -14,6 +14,7 @@
 #define WSREP_SERVER_SERVICE_HPP
 
 #include "logger.hpp"
+#include "server_state.hpp"
 
 #include <string>
 
@@ -93,6 +94,20 @@ namespace wsrep
          */
         virtual void log_view(const wsrep::view&) = 0;
 
+        /**
+         * Log a state change event.
+         *
+         * Note that this method may be called with server_state
+         * mutex locked, so calling server_state public methods
+         * should be avoided from within this call.
+         *
+         * @param prev_state Previous state server was in
+         * @param current_state Current state
+         */
+        virtual void log_state_change(
+            enum wsrep::server_state::state prev_state,
+            enum wsrep::server_state::state current_state) = 0;
+
         /**
          * Determine if the configured SST method requires SST to be
          * performed before DBMS storage engine initialization.