Performance optimizations

Each topic now has its own ekaf_server_<topic> pid that only picks
a new worker if needed every second. Messages in Queue drastically reduced when hit with millions of requests over long periods of time.

Refactored pop_messages_from_queue to flush. Added {ekaf_callback_flush,{M,F}}
that can be used for instrumenting
Better first handling of topic, added screenshot of eunit and kafka consumer