Cache
- add dependency for metadata renderer file
- check metadata for end of page life, "date valid end".

Metadata Renderer
- RSS syntax mode now sets rendered page expiry, "date valid end"
  and includes the feed URL in "relation haspart".

Purgefile
For all wiki installations the purgefile records the earliest
time before which no cache purge (based on data consistency)
is required. Cache files older than this time MAY need to be
purged.
- remove purgeonadd configuration setting

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