sources: update reference leap status early

When a leap second status is updated by a source, don't wait for the
next source selection and full update of the reference. Count votes from
sources that passed the previous selection and update the reference leap
status directly.

This should allow leap seconds to spread quickly even when the
samples are dropped or delayed by the filters.

reference.h

@@ -162,6 +162,9 @@ extern void REF_SetManualReference
 extern void
 REF_SetUnsynchronised(void);
 
+/* Announce a leap second before the full reference update */
+extern void REF_UpdateLeapStatus(NTP_Leap leap);
+
 /* Return the current stratum of this host or 16 if the host is not
    synchronised */
 extern int REF_GetOurStratum(void);