parser caching update

This patch primarily updates p_cached_xhtml() and p_cached_instructions() to
allow their caching logic to be surrounded by an event trigger.

p_cached_xhtml() has been rewritten as the more general p_cached_output() to
support other render output formats besides 'xhtml'. All calls to
p_cached_xhtml() have been changed to refer to the new function.

New event:

name:        PARSER_CACHE_USE
data:        cache object (see below)
action:      determine if cache file can be used
preventable: yes
result:      bool, true to use cache file, false otherwise

Cache operations have been generalised in a new class, cache, extended to
cache_parser, cache_renderer & cache_instructions. Details can be found in
inc/cache.php

For handling of above event, key properties are:
- page, if present the wiki page id,
        may not always be present, e.g. when called for locale xhtml files
- file, source file
- mode, renderer mode (e.g. 'xhtml') or 'i' for instructions

Other changes:
- cache class counts cache hits against attempts, results are stored in
  {cache_dir}/cache_stats.txt
- adds metadata dependency to renderer page cache
- replaces purgefile dependency for renderer cache with metadata
  'relation references' (internal link) dependency for wiki pages only

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