JIRA: https://jira.spring.io/browse/INT-4349
When headers come from the external system there is no guarantee
that special headers (e.g. `sequenceNumber`, `priority` etc.) in the
expected (`Integer`) type.
* Widen `int` headers setting value to the `Number` type
* Return primitive `int` for the `sequenceNumber` and `sequenceSize`
headers since for them `IntegrationMessageHeaderAccessor` never return
null for them
* Remove `SequenceNumberComparator` in favor of `MessageSequenceComparator`
since they are essentially duplicate each other
* Fix tests to deal with primitive `int` already
c65584a007