make slow_callback_duration a parameter in asyncio debugging

[imLew]

Proposed changes:

• Add the parameter slow_callback_duration to the utils function enable_async_loop_debugging and change its default value to 100ms, up from 1ms.
  Any asyncio calls that take longer than slow_callback_duration produce a warning. (There were lots of warnings)
  That was also the reason agent.load_model_from_server called set_log_level, since this is no longer necessary the call has been removed (it caused some other troubles).

enable_async_loop_debugging is called in tests and if rasa is run with --debug. A default value of 100ms might not always be appropriate and we might have to change it in some places for #4356

Status (please check what you already did):

• made PR ready for code review
• added some tests for the functionality
• updated the documentation
• updated the changelog
• reformat files using black (please check Readme for instructions)