Start optional object cache for parsed and preprocessed template posts

[eliot-akira]

This pull request implements an optional object cache for parsed template posts. And in the future possibly other pre-processed and cached data.

It's similar in direction to the previous prototype except:

• Implemented as an admin feature of the template system - instead of in the HTML parser
• Only caches template posts - instead of every template rendered
• Refresh cache on post save, and use post type and ID for cache key - instead of creating and comparing a hash of template content

Status: Working but experimental, needs testing and real-life usage

• Resolves Add option to turn on experimental caching #127

@GabrielGallagher @titus-toia

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Use the Transients API if you need to guarantee that your data will be cached. If persistent caching is configured, then the transients functions will use the wp_cache_* functions described in this document. However if persistent caching has not been enabled, then the data will instead be cached to the options table.

• https://developer.wordpress.org/apis/transients
• https://developer.wordpress.org/reference/classes/wp_object_cache/
• https://developer.wordpress.org/advanced-administration/performance/cache/