Investigate the performance limits and controls of getdns

[saradickinson]

Observations from the IETF demo showed a threshold in the number of messages that could be sent in async mode before seeing multiple timeouts. This needs investigation.

Also need to consider the right algorithm to use to implement the getdns_context_set_limit_outstanding_queries() method. Wouter commented that using a sharp cut-off when reaching the limit is often interpreted as users as the same as a failure.

[saradickinson]

I am going to close this because I believe the work done recently on the event loops, scheduling and switch to poll() have addressed the main issues. Also @wtoorop has implemented getdns_context_set_limit_outstanding_queries()