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
```

Current implementation is not optimal. It is using fixed-size go-routines pool. If the pool specified by the user is too small, one slow resource could block processing of other resources (different keys, but same hash). User can adjust the pool size, but it is very hard to figure out this number upfront (before running the app on production).

The new implementation spawn a dedicated go-routine for each new batch. At max one go-routine is created for given resource key. Go-routine is destroyed once batch ends.