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
