paycore: Prevent multiple concurrent payment groups

One of the fundamental constraints of the payment groups idea is that
there may only ever be one group in flight at any point in time, so if
we find a group that is in flight, any new `sendpay` or `sendonion`
must match its `groupid`.

lightningd/pay.c

@@ -910,12 +910,24 @@ send_payment_core(struct lightningd *ld,
 			return sendpay_success(cmd, payments[i]);
 
 		case PAYMENT_PENDING:
+			/* At most one payment group can be in-flight at any
+			 * time. */
+			if (payments[i]->groupid != group) {
+				return command_fail(
+				    cmd, PAY_IN_PROGRESS,
+				    "Payment with groupid=%" PRIu64
+				    " still in progress, cannot retry before "
+				    "that completes.",
+				    payments[i]->groupid);
+			}
+
 			/* Can't mix non-parallel and parallel payments! */
 			if (!payments[i]->partid != !partid) {
 				return command_fail(cmd, PAY_IN_PROGRESS,
 						    "Already have %s payment in progress",
 						    payments[i]->partid ? "parallel" : "non-parallel");
 			}
+
 			if (payments[i]->partid == partid) {
 				/* You can't change details while it's pending */
 				if (!amount_msat_eq(payments[i]->msatoshi, msat)) {