SegmentId PR

[Roman Leventov]

Could somebody please provide design review of "Introduce SegmentId class"
PR : https://github.com/apache/incubator-druid/pull/6370? This is an
important improvement, and many other improvements and bugs fixes are
blocked on it. Despite "Development Blocker" tag (that was thought to give
PRs a priority), nobody reviewed this PR for almost two months, except Egor
with whom we work for the same company.