MetadataSegmentView should correctly handle the situation of Coordinator being down or changed leadership

[leventov]

Regardless of whether any of the designs proposed for MetadataSegmentView here: #6834 (comment) is applied, MetadataSegmentView should document its behavior in the face of the Coordinator being down, or just started, or leadership changed.

Currently, there is a single comment "Note that coordinator must be up to get segments" which, IMO, only adds FUD and doesn't explain anything.

Ideally, MetadataSegmentView should correctly recognize the situations of changed Coordinator's leadership and not to flush all cached published segments.

Explanation may look like the following:

/**
 * When Coordinator is just started or leadership changed,
 * its SegmentsMetadata is unpopulated. Then the endpoint /metadata/abc/foo/bar
 * which we are querying returns code XXX (see {@link DataSourcesResource#theRelevantMethod}).
 * We process this code here and don't flush {@link #cachedPublishedSegments} in this case.
 */

FYI @surekhasaharan

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