Possibility to disable the 'No subscribers registered for event' log messages

[mreichelt]

When posting an event that currently has no subscribers EventBus will log a 'No subscribers registered for event' message.
This should be configurable, or even better: be disabled by default.

[greenrobot]

Added checkConfigurationAllowed() to enforce consistent behavior. Thanks!

[pizza]

@greenrobot I can't find this anymore in the latest version of the code. Was there a reason why it was removed?

[greenrobot]

@pizza Check EventBusBuilder