This won't scale for a large number of messages; it also won't retain FIFO within priority. It would be better to use a TreeMap, perhaps keyed by `priority + an incrementing integer` (to retain insertion order). We really need a `LinkedTreeMap`.

However, this would still be flawed because we need the underlying MessageStore to retain the FIFO semantics (within priority) too so that we get the same order when initializing from the store.

This is why FIFO code was added to the basic `PriorityChannel`. See INT-2031, INT-2544, INT-2508.