[trac678] Make sure io_service stops when asked to

.stop() followed directly by .reset() doesn't work. It seems asio has a
"can run" boolean variable, stop sets it to false, reset to true, so if
they are both called from inside the .run() main loop, it does nothing.

src/lib/asiolink/tests/dns_server_unittest.cc

@@ -367,7 +367,6 @@ class DNSServerTest : public::testing::Test {
         static void stopIOService(int _no_use_parameter) {
             io_service_is_time_out = true;
             service.stop();
-            service.reset();
         }
 
         bool serverStopSucceed() const {