Make is_rollbacker_active() public, don't unlock for background rollback

Make is_rollbacker_active() public so that the BF thread can
check if the rollbacker was started or not.

Also don't unlock the lock for launching the background
rollbacker to avoid race conditions in accessing the
victim state.

src/transaction.cpp

@@ -1073,9 +1073,7 @@ bool wsrep::transaction::bf_abort(
                 }
             }
 
-            lock.unlock();
             server_service_.background_rollback(client_state_);
-            lock.lock();
         }
     }
     return ret;