Draft: Make worker->shuttingdown atomic
This worker->shuttingdown
could be accessed concurrently from within the
context of multiple threads. For example, it is used in
isc__nmsocket_closing()
, which can be called on listener sockets from
a context of a thread which does not own the object.
This lead to TSAN reports in the Stream DNS branch (!6707 (merged)), this commit should fix that.