Spatial CellTokenStream re-use [LUCENE-5776]

It would be nice if CellTokenStreams were re-used, which are kinda heavy. This is possible without changing the API thanks to #6696. PrefixTreeStrategy.createFields() should return Field subclasses that override `TokenStream tokenStream(Analyzer analyzer, TokenStream reuse)` that in turn examine re-use to ensure it's a CellTokenStream, and then re-use it if possible or create anew.

Completing the re-use, it would be good if TreeCellIterator could be re-used, which might include the getWorldCell value.

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Migrated from [LUCENE-5776](https://issues.apache.org/jira/browse/LUCENE-5776) by David Smiley (@dsmiley)

Start by tracing PrefixTreeStrategy.createFields() and the Field subclasses it returns, then inspect TokenStream tokenStream(Analyzer analyzer, TokenStream reuse) and CellTokenStream. Next, examine TreeCellIterator, including getWorldCell; done means compatible CellTokenStream and TreeCellIterator instances are reused without changing the API.

Written by the indexing model from the issue text.