Allow the async rendering framework to limit the pool of threads based on AsyncRenderer type

The Async rendering framework allows to create jobs with a unique ID and to answer later the result of the job. So right now, async rendering is based on the Job executor to execute the threads, and our default implementation of Job executor is to use a pool with the maximum size possible (Integer.MAX_VALUE in DefaultJobExecutor for the construction of the pool).

It would be very useful to have a more fine-grained capability to be able to set the maximum number of threads for each kind of Async renderer. For example, we don't want to have a large number of threads for computing the count notifications.