Adjust refclock filter parameters

Drop only about 40 percent of samples, change default length to 64,
require at least 4 samples between polls (or full filter for lengths
below 4).

conf.c

@@ -448,7 +448,7 @@ parse_refclock(const char *line)
 
   poll = 4;
   dpoll = 0;
-  filter_length = 15;
+  filter_length = 64;
   pps_rate = 0;
   offset = 0.0;
   delay = 1e-9;