sources: allow setting minsamples and maxsamples for each source

The minsamples and maxsamples directives now set the default value,
which can be overriden for individual sources in the server/peer/pool
and refclock directives.

refclock.h

@@ -38,6 +38,8 @@ typedef struct {
   int poll;
   int filter_length;
   int pps_rate;
+  int min_samples;
+  int max_samples;
   uint32_t ref_id;
   uint32_t lock_ref_id;
   double offset;