Make query chain processing asynchronous

Make query chain processing asynchronous

Under some circumstances when processing a query response - for example,
when it contains a CNAME or DNAME - a query will have to be restarted
from the beginning to look up a new target.

This was previously handled by recursively calling the ns__query_start()
function directly from ns_query_done(). However, performance test data
indicated that chains of CNAMEs could consume quite a bit of time inside
the worker thread, increasing latency for other waiting queries.  This
has now been changed so that restarted queries are run asynchronously.