Speed up in the search cache by avoiding construction of the set of all book ids when the cache hits

Merge branch 'master' of https://github.com/cbhaley/calibre
Tentative implementation of using global render cache when evaluating composite columns in composite_getter()
Refactor to make the template rendering logic its own function
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Make render_composite() an internal API
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Improve performance of get_categoris by using ProxyMetadata objects