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).
This commit contains three changes in the three files:
1. A bug in the backup process meant that anything logged after a
certain point did not appear in the on-screen display of the
backup log, because the logger was serialised and deserialised
but display code referred to the old version. Changed so that
code retrieves new object.
2. Add more information to backup log when there is a missing file.
3. Add more information to restore log when there is a missing file
(and remove existing code duplication of the current message).
The 'missing file' situation is one that generally shouldn't occur in
normal usage, but when it does happen, it is useful to have full
information about the file.
