[Notification Service] Prevent mass mail outbursts from defective prosEO service #246
Description
When a prosEO service goes "berserk" (e. g. looping on an error), it may send thousands of notification requests within a very short time span. The Notification Service shall be extended to detect such an outburst and after some configurable limit prevent the sending of further notification mails, until the outburst subsides. When mail sending for the source is resumed, a log message shall state, how many messages were dropped.