Add method client_state::stream()

This method forces a streaming replication step.

include/wsrep/client_state.hpp

@@ -354,6 +354,14 @@ namespace wsrep
          */
         int after_row();
 
+        /**
+         * Force a streaming step
+         *
+         * This method can be used to replicate a fragment in the
+         * current context.
+         */
+        int stream();
+
         /**
          * Set streaming parameters.
          *