This changes the cache logic for metadata. It introduces a new mode that
tries to avoid rendering the page again for simple requests but still
updates the metadata when the page has been changed (but not when the
cache timeout has been reached or purge is used). It simply compares the
time of the last rendering with the last modified time of the page.

The old boolean $render parameter has been changed into an int with
three possible values. Compatibility for the old parameter is provided
using a check with is_numeric using the following mapping:
- false is still don't render (0 is the new value for that)
- true is using that new render logic which means that many plugins will
  still work unchanged even if they request a lot of data using
  $render=true (1 is the new value for that providing full compatibility
  in the case 1 has been used instead of true)

The default value for p_get_first_heading is now that new simple cache
logic, the default value for getting metadata is the cache logic which
should be used with care but is the only way to request (rendered)
metadata that can change because of plugin installations or upgrades.