Client code executing Processor.Run might want to abort running operation if the operation was not yet run in a batch. Such situation is possible when there is a high request congestion.

From now on Processor.Run will accept new parameter context.Context. This context could be cancelled by the client effectively dropping the operation if it was still waiting to be run.

Example:

```
ctx := context.WithTimeout(context.Background(), 5 * time.Second)
err := processor.Run(ctx, "key", ...)
// err will be OperationCancelled
```