pay: fix problematic sharing of shadow route data.

Because listchannels responses are async, with mpp we can end up
fighting over use of the parent's data giving results from incorrect
final CLTVs to assertion failures like below:

```
pay: plugins/libplugin-pay.c:1964: shadow_route_listchannels: Assertion `amount_msat_greater_eq(p->constraints.fee_budget, d->constraints.fee_budget)' failed.
pay: FATAL SIGNAL 6 (version v0.9.0rc2-11-g29d32e0-modded)
0x563129eb6a15 send_backtrace
	common/daemon.c:38
0x563129eb6abf crashdump
	common/daemon.c:51
0x7fe37c8b920f ???
	???:0
0x7fe37c8b918b ???
	???:0
0x7fe37c898858 ???
	???:0
0x7fe37c898728 ???
	???:0
0x7fe37c8a9f35 ???
	???:0
0x563129ea5c63 shadow_route_listchannels
	plugins/libplugin-pay.c:1964
0x563129e9c56a handle_rpc_reply
	plugins/libplugin.c:547
0x563129e9cbfa rpc_read_response_one
	plugins/libplugin.c:662
0x563129e9ccf0 rpc_conn_read_response
	plugins/libplugin.c:681
0x563129ece660 next_plan
	ccan/ccan/io/io.c:59
0x563129ecf245 do_plan
	ccan/ccan/io/io.c:407
0x563129ecf287 io_ready
	ccan/ccan/io/io.c:417
0x563129ed151f io_loop
	ccan/ccan/io/poll.c:445
0x563129e9ef03 plugin_main
	plugins/libplugin.c:1284
0x563129e9b099 main
	plugins/pay.c:2010
0x7fe37c89a0b2 ???
	???:0
0x563129e9452d ???
	???:0
0xffffffffffffffff ???
	???:0
```

plugins/libplugin-pay.c

@@ -1750,13 +1750,21 @@ REGISTER_PAYMENT_MODIFIER(exemptfee, struct exemptfee_data *,
 
 static struct shadow_route_data *shadow_route_init(struct payment *p)
 {
+	struct shadow_route_data *d = tal(p, struct shadow_route_data), *pd;
+
+	/* If we're not the root we need to inherit the flags set only on the
+	 * root payment. Since we inherit them at each step it's sufficient to
+	 * do so from our direct parent. */
 	if (p->parent != NULL) {
-		return payment_mod_shadowroute_get_data(p->parent);
+		pd = payment_mod_shadowroute_get_data(p->parent);
+		d->fuzz_amount = pd->fuzz_amount;
+#if DEVELOPER
+		d->use_shadow = pd->use_shadow;
+#endif
 	} else {
-		struct shadow_route_data *d = tal(p, struct shadow_route_data);
 		d->fuzz_amount = true;
-		return d;
 	}
+	return d;
 }
 
 /* Mutual recursion */