Support for changing FieldCache implementation in Lucene [LUCENE-5427]

Believe in general it would be a nice feature to be able to overwrite the FieldCache-implementation in Lucene.

For instance we override with a FieldCache-implementation that basically does not allow anything to be put in the FieldCache caches. We have so much data that it always creates memory issues.

But I could imagine other reasons for wanting to override the FieldCache-implementation

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Migrated from [LUCENE-5427](https://issues.apache.org/jira/browse/LUCENE-5427) by Per Steffensen (@steff1193), 2 votes
Attachments: [Override_FieldCache_impl_to_use.patch](https://apache.github.io/lucene-jira-archive/attachments/LUCENE-5427/Override_FieldCache_impl_to_use.patch)
Linked issues:
- [SOLR-5686](https://issues.apache.org/jira/browse/SOLR-5686)

Start by reviewing the attached Override_FieldCache_impl_to_use.patch and the linked SOLR-5686 issue to understand the proposed extension point and its existing context. Trace the relevant FieldCache implementation and tests in the Lucene source, then define completion as allowing the implementation to be overridden without the current cache behavior while preserving existing functionality.

Written by the indexing model from the issue text.