Property: fix intermittent test failure.
The reduction in the cache flush threshold in caused the stochastic test
to fail with noticeable probability.  Revert that part of the change.
Also add a comment to help avoid this in future.
#include "crypto/sparse_array.h"
#include "property_local.h"
-/* The number of elements in the query cache before we initiate a flush */
-#define IMPL_CACHE_FLUSH_THRESHOLD 50
+/*
+ * The number of elements in the query cache before we initiate a flush.
+ * If reducing this, also ensure the stochastic test in test/property_test.c
+ * isn't likely to fail.
+ */
+#define IMPL_CACHE_FLUSH_THRESHOLD 500
typedef struct {
void *method;
