retrymod: Make retry modifier slightly more verbose

I found it rather useful to trace how a payment is getting retried in the logs.

plugins/libplugin-pay.c

@@ -1377,6 +1377,14 @@ static inline void retry_step_cb(struct retry_mod_data *rd,
 		subpayment->why =
 		    tal_fmt(subpayment, "Still have %d attempts left",
 			    rdata->retries - 1);
+		plugin_log(
+		    p->plugin, LOG_DBG,
+		    "Retrying %s/%d (%s), new partid %d. %d attempts left\n",
+		    type_to_string(tmpctx, struct sha256, p->payment_hash),
+		    p->partid,
+		    type_to_string(tmpctx, struct amount_msat, &p->amount),
+		    subpayment->partid,
+		    rdata->retries - 1);
 	}
 
 	payment_continue(p);