refclock: make maximum lock age configurable

The maxlockage option specifies in number of pulses how old can be
samples from the refclock specified by the lock option to be paired with
the pulses. Increasing this value is useful when the samples are
produced at a lower rate than the pulses.

refclock.h

@@ -41,6 +41,7 @@ typedef struct {
   int min_samples;
   int max_samples;
   int sel_options;
+  int max_lock_age;
   uint32_t ref_id;
   uint32_t lock_ref_id;
   double offset;