ExecutorService can be used instead of ScheduledExecutorService

The MqttAsyncClient needs a ScheduledExecutorService param for the constructor but only the functionality of a ExecutorService is needed. Other frameworks works more often with ExecutorService and provides good functionality to create such a service. Based on all that points ExecutorService should be used instead.

Start at the MqttAsyncClient constructor and trace every use of its ScheduledExecutorService parameter. Check the existing tests and public API expectations before assessing the change; done means the client needs only ExecutorService functionality without breaking supported construction paths.

Written by the indexing model from the issue text.