codership/wsrep-lib#23 before_command() wait for ongoing rollbacks leaks

Storing information that background rollbacker in ongoing in client state has_rollback_
This can be used for detecting if there is ongoing background rollback,
and client should keep waiting in before_command() entry to avoid conflicts
in accessing client state during background rollbacking.

transaction::bf_abort() is modified to set has_rollback_ flag when
backgroung rollbacking has been assigned for the client

sync_rollback_complete() method has been modified to reset the backround
rollbacker flag

test/test_utils.hpp

@@ -43,5 +43,4 @@ namespace wsrep_test
     void bf_abort_provider(wsrep::mock_server_state& sc,
                            const wsrep::transaction& tc,
                            wsrep::seqno bf_seqno);
-
 }