Send Client Report When Fully Rate Limited

[philipphofmann]

Description

The Cocoa SDK only attaches a client report envelope item when there are envelope items to send after applying the rate limit. This means, that the SDK won't send out a client report if all envelope items are rate limited or for example the user drops all events in beforeSend. We can fix this by checking in the HTTPTransport if the ClientReports contain more than x discarded events and then send out the client report and by only sending out a client report maximum every x seconds.