Enhance UpgradeIndexMergePolicy with additional options [LUCENE-7671]

Enhance UpgradeIndexMergePolicy to be a MergePolicy that can be used outside the scope the IndexUpgrader.

The enhancement aims to allow the UpgradeIndexMergePolicy to:

1) Delegate normal force merges to the underlying merge policy
2) Enable a flag that will explicitly tell UpgradeIndexMergePolicy when it should start looking for upgrades.
3) Allow new segments to be considered to be merged with old segments, depending on underlying MergePolicy.
4) Be configurable for backwards compatibility such that only segments needing an upgrade would be considered when merging, no explicit upgrades.

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Migrated from [LUCENE-7671](https://issues.apache.org/jira/browse/LUCENE-7671) by Keith Laban, updated Apr 22 2018
Linked issues:
- [SOLR-12259](https://issues.apache.org/jira/browse/SOLR-12259)

Start with UpgradeIndexMergePolicy and the underlying MergePolicy it wraps, then review the linked SOLR-12259 context. Compare the requested delegation, upgrade flag, segment-mixing, and backwards-compatibility behaviors. Done means these options are defined and validated for the stated use cases, including use outside IndexUpgrader.

Written by the indexing model from the issue text.