sourcestats: add fixed minimum delay

If the minimum delay is known (in a static network configuration), it
can replace the measured minimum from the register. This should improve
the stability of corrections for asymmetric jitter, sample weighting and
maxdelay* tests.

test/unit/sources.c

@@ -53,7 +53,7 @@ test_unit(void)
 
       DEBUG_LOG("added source %d options %d", j, sel_options);
       srcs[j] = SRC_CreateNewInstance(UTI_IPToRefid(&addr), SRC_NTP, sel_options, &addr,
-                                         SRC_DEFAULT_MINSAMPLES, SRC_DEFAULT_MAXSAMPLES);
+                                      SRC_DEFAULT_MINSAMPLES, SRC_DEFAULT_MAXSAMPLES, 0.0);
       SRC_UpdateReachability(srcs[j], 1);
 
       samples = (i + j) % 5 + 3;