Refactored parts of the new ZGC parsing API to improve consistency

[fthevenet]

Hi,

I would like to propose a few changes to the way some classes and class members are named in the ZGC parsing API.
Since #447 introduced a breaking change in this part of the API and has not yet be released, I think this is a good time to review a few inconsistencies that make discovery of the API more difficult.

Changes can be grouped as such:

1. Changed incorrect references to "ReclaimStart/End" instead of "RelocateStart/End" in ZGCReclaimSummary
2. Renamed occurrences of the term "Reclaim" to "Reclaimed" in many places, for consistency.
3. Renamed the class OccupancySummary to ZGCUsedSummary for consistency
4. Renamed methods get/setGenerationUsedSummary to get/setUsedSummary for consistency.

Thanks.

[fthevenet]

@microsoft-github-policy-service agree company="IBM"