quantiles: add parameter to limit negative step

Add a new parameter to limit the negative value of the step state
variable. It's set as a maximum delay in number of updates before the
actual step applied to the quantile estimate starts growing from the
minimum step when the input value is consistently larger or smaller than
the estimate.

This prevents the algorithm from effectively becoming the slower 1U
variant if the quantile estimate is stable most of the time.

Set it to 100 updates for the NTP delay and 1000 updates for the hwclock
delay. An option could be added later to make it configurable.

hwclock.c

@@ -49,6 +49,7 @@
 #define DELAY_QUANT_MAX_K 2
 #define DELAY_QUANT_Q 10
 #define DELAY_QUANT_REPEAT 7
+#define DELAY_QUANT_LARGE_STEP_DELAY 1000
 #define DELAY_QUANT_MIN_STEP 1.0e-9
 
 struct HCL_Instance_Record {
@@ -127,6 +128,7 @@ HCL_CreateInstance(int min_samples, int max_samples, double min_separation, doub
   clock->precision = precision;
   clock->delay_quants = QNT_CreateInstance(DELAY_QUANT_MIN_K, DELAY_QUANT_MAX_K,
                                            DELAY_QUANT_Q, DELAY_QUANT_REPEAT,
+                                           DELAY_QUANT_LARGE_STEP_DELAY,
                                            DELAY_QUANT_MIN_STEP);
 
   LCL_AddParameterChangeHandler(handle_slew, clock);