Fix longstanding issue with cache class & cachetime setting

1. cachetime setting should only be applied to the cache_renderer
   class.  Previously it was applied to cache_parser (and by extension cache_instructions).
2. two special cachetime values, -1 & 0, weren't handled, per
   [FS#2183](https://bugs.dokuwiki.org/index.php?do=details&task_id=2183&project=1)

To handle the cachetime setting, -1, disable caching, a new
property _nocache is added.  When that property is true, any cache
file must not be used and storecache() should not store any values