The completion system has code which is supposed to prevent it
updating completion status while grades are being restored. This
code was a nasty hack and had not worked for some time. As a result
completion dates were restored incorrectly.
This commit implements a 'proper' way to find out if a restore is
currently running, replacing the previous hack.
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.
