Outstanding responses for consumed commands in connections should throttle the consumption of messages from the source

[thjaeckle]

During a session with @alstanchev we found out that for Ditto managed Kafka connections (probably same for other backpressuring connections) when

• signals (e.g. "ModifyThing" commands) are processed
• which require a response (responseRequired=true- which is also the default) to be send to a configured "replyTarget" address on the same Kafka
• if those expected replies can not be delivered quickly enough
• the consumption of new messages (from the connection "source") will not backpressure and consume slower from Kafka, but it will drop responses with warn message "Message is dropped as a result of backpressure strategy"

That was some kind of unexpected for us both - the connection should be well aware that it cannot deliver responses quickly enough and should slow down consumption of messages.