ntp: use async name resolving for NTP sources

Use the new asynchronous call to resolve addresses of NTP servers
configured by the server/peer directives. Introduce a callback to be
notified when the first resolving attempt ends to correctly finish
chronyd initialization (dumpfile reload and reference mode end).

conf.c

@@ -1186,8 +1186,6 @@ CNF_AddSources(void) {
     NSR_AddUnresolvedSource(ntp_sources[i].params.name, ntp_sources[i].params.port,
         ntp_sources[i].type, &ntp_sources[i].params.params);
   }
-
-  NSR_ResolveSources();
 }
 
 /* ================================================== */