review experimental classes [LUCENE-4325]

This can frighten people away from APIs (maybe as much as `@deprecated`).

We should be careful what we tag as experimental: things like Collector which have been around for an aweful long time imo should not be, its ok if they are expertish, if we need to make a change we take that on a case by case basis.

We also don't need this tag on pkg-private things.

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Migrated from [LUCENE-4325](https://issues.apache.org/jira/browse/LUCENE-4325) by Robert Muir (@rmuir)
Attachments: [LUCENE-4325_start.patch](https://apache.github.io/lucene-jira-archive/attachments/LUCENE-4325/LUCENE-4325_start.patch)

Start by reviewing the attached LUCENE-4325_start.patch and the classes it changes. Determine which experimental tags are unnecessary for long-standing public APIs or package-private code, and document the resulting review decisions. The issue does not name specific files or tests, so completion criteria require project familiarity.

Written by the indexing model from the issue text.