encode_backup_temp_info() and decode_backup_temp_info() have been
introduced to keep the info field encoding in one place.
Many locations used get_backup_ids_record() to obtain info, that
makes lots of calls to get_backup_ids_record() which can be slow.
We now complete all those inline by adding the info field to the query.
To reduce memory usage, all queries of that nature have been changed
to use get_recordset_*. gzcompress was introduced if available to minimize
traffic to/from the database and to decrease the memory required for caching.
The compression time is saved by the benefits in other places of having smaller data.
When we import courses or duplicate activities, the deduplicating nature of
the file_storage system means that we don't actually have to include the
real files in the backup - just their metadata from the file table.
The restoration process needs to know not to expect files from the backup
phase so a flag is set in moodle_backup metadata.
Russell Smith