src/meta_data.[ch]: Added a framework for meta data.

src/meta_data.[ch]: Added a framework for meta data.

This framework is planned to be used in at least three places:

- Extend the global cache to allow attaching a meta-data object to each
  entry. The meta-data can then be used by other plugins to attach
  miscellaneous information to an entry. Possibly uses include
  calculation of floating averages or storing the state for a hysteresis
  threshold.

- Add a meta-data object to every `value_list_t' in
  `plugin_dispatch_values'. The meta-data object can then be used by
  matches and targets to pass information about a value-list to one
  another.

- Replace the notification meta-data stuff in src/plugin.[ch] with such
  an object.