ntp: avoid unnecessary restart of resolving round on refresh

Don't call NSR_ResolveSources() when a resolving round is already
started. This cuts the number of calls of the system resolver made due
to the refresh command to half.

ntp_sources.c

@@ -1056,6 +1056,9 @@ resolve_source_replacement(SourceRecord *record, int refreshment)
   us->address = *record->remote_addr;
 
   append_unresolved_source(us);
+
+  /* Don't restart resolving round if already running */
+  if (!resolving_source)
     NSR_ResolveSources();
 }