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/gtid.hpp

@@ -48,6 +48,13 @@ namespace wsrep
         {
             return undefined_;
         }
+        bool operator==(const gtid& other) const
+        {
+            return (
+                seqno_ == other.seqno_ &&
+                id_ == other.id_
+            );
+        }
     private:
         static const wsrep::gtid undefined_;
         wsrep::id id_;