created arg recursion_count to async_resolve and many other to avoid infinite loops

created arg recursion_count to async_resolve and many other to avoid infinite loops

* arg recursion_count replaces cname_count which up to now identified problems in post-processing
* created simple loop test
* added option to configure recursion depth limit to DnsMaster

minor other changes:
* corrected docu for resolved-ip-ttl-threshold option: is not milliseconds but seconds
* add test to ResolverBase: check number of callbacks and warn if suspiciously many