Log noisy messages at DEBUG level

[jystephan]

Is your feature request related to a problem? Please describe.
I've set my Delight log level to INFO, this is important for me as I want to see the URL at which my Delight applications are accessible.

But I'm getting flooded with noisy log messages like:

• s"Flushing ${eventsBuffer.length} events, now ${pendingEvents.size} pending events"
• Successfully sent heartbeat
• Successfully sent payload
• Successfully sent ack

Describe the solution you'd like
No INFO logs should be sent in a periodic thread while the application is running. They should be log at DEBUG.

Describe alternatives you've considered
Here's the workaround implemented by one of Delight users. He has dynamically set the log level of Delight to ERROR a few seconds after the Spark session initialization, directly in his Spark code, using:

log4j.LogManager.getLogger("co.datamechanics.delight").setLevel(log4j.Level.ERROR)

This way he had the INFO logs at Delight initialization, but not the noisy logs afterwards. It's just a workaround.