KAFKA-18080: Replace DelayedItem by Long type

[Yunyung]

Remove DelayedItem and update the code accordingly.

JIRA: https://issues.apache.org/jira/browse/KAFKA-18080

Committer Checklist (excluded from commit message)

• Verify design and implementation
• Verify test coverage and CI build status
• Verify documentation (including upgrade notes)

[chia7712]

@Yunyung Could you please remove DelayedItem directly? the only use case is PartitionFetchState and so we can pass dueMs: Option[Long] instead of delay Option[DelayedItem]. The method isDelayed can be replaced by following code.

def isDelayed: Boolean = dueMs.map(_ - Time.SYSTEM.milliseconds).exists(_ > 0)

[Yunyung]

Thank you! I removed DelayedItem and updated the only use case, PartitionFetchState, accordingly.

[Yunyung]

Fix the typo I found in the code comment for PartitionFetchState.

[chia7712]

LGTM