gossipd: increase randomness in route selection.

We have a seed, which is for (future!) unit testing consistency.  This
makes it change every time, so our pay_direct_test is more useful.

I tried restarting the noed around the loop, but it tended to fail
rebinding to the same port for some reason?

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

common/pseudorand.c

@@ -37,6 +37,13 @@ uint64_t pseudorand(uint64_t max)
 	return isaac64_next_uint(&isaac64, max);
 }
 
+uint64_t pseudorand_u64(void)
+{
+	init_if_needed();
+
+	return isaac64_next_uint64(&isaac64);
+}
+
 const struct siphash_seed *siphash_seed(void)
 {
 	init_if_needed();