Add delayed name resolving for servers and peers

Resolving is retried in increasing intervals (maximum is one hour)
until it succeeds or fails with a non-temporary error.

Unresolved sources are included in the activity report as offline
sources and the online command can be used to retry it immediately.

This could be improved by resolving in a separate thread/process
to avoid blocking.

cmdparse.h

@@ -49,6 +49,7 @@ typedef enum {
 
 typedef struct {
   IPAddr ip_addr;
+  char *name;
   unsigned short port;
   SourceParameters params;
 } CPS_NTP_Source;