Using standard subplugin support, this commit implements
the backup of logstore subplugins in general and the
standard logstore in particular. Notes:

- Uses a custom final element (base64_encode_final_element) to
  support the storage of serialized 'other' information in logs.
- Organization: Instead of directly extending backup_subplugin,
  every logstore extends backup_tool_log_logstore_subplugin just
  in case any shared code is needed in the future.
- Implements both course and activity logs, sharing the structure
  completely (both are based in contextid to pick the target
  information, from database or whatever other logstores use).