TrackerQueue window pointers could wrap. Increase numeric range or use modular arithmatic

TrackerQueue uses "int" data type for numbering deliveries and managing the window size.
tests are not coded for circular arithmetic so weirdness will start after 2^31 deliveries.
Quick fix is to make these labels long.
Slightly less quick fix is to use circular arithmetic.
in getDelivery()
if (seq < _lwm || seq > _hwm) return null;
change to
if (((seq - _lwm) < 0) || ((seq - _hwm) > 0)) return null;

in apply()
if (seq < _lwm || seq > _hwm) return;
change as above.