codership/wsrep-lib#104 Error voting support

- populate and pass real error description buffer to provider in case
   of applying error
 - return 0 from server_state::on_apply() if error voting confirmed
   consistency
 - remove fragments and rollback after fragment applying failure
 - always release streaming applier on commit or rollback

include/wsrep/client_id.hpp

@@ -41,6 +41,10 @@ namespace wsrep
         {
             return (id_ < other.id_);
         }
+        bool operator==(const client_id& other) const
+        {
+            return (id_ == other.id_);
+        }
     private:
         type id_;
     };