accesslib: Introduce functions to deal with dirty contexts

accesslib: Introduce functions to deal with dirty contexts

The accessinfo held in $USER->access can easily get out of
sync with reality if and admin has removed our access,
or expanded it after we loaded our accessinfo.

To handle this, we'll use the config_plugins table with an
'accesslib/dirtycontexts' plugin signature to store the paths of
recently changed contexts. To handle those dirrrty entries, here
we introduce

  get_dirty_contexts() - for lib/setup
  mark_context_dirty()
  cleanup_dirty_contexts() - for cron