ntp: add filter option

Add an option to use the median filter to reduce noise in measurements
before they are accumulated to sourcestats, similarly to reference
clocks. The option specifies how many samples are reduced to a single
sample.

The filter is intended to be used with very short polling intervals in
local networks where it is acceptable to generate a lot of NTP traffic.

srcparams.h

@@ -49,6 +49,7 @@ typedef struct {
   int max_sources;
   int min_samples;
   int max_samples;
+  int filter_length;
   int interleaved;
   int sel_options;
   uint32_t authkey;