cleanup logging for BackupController.purge

Move the logging message for "Purging" after the release
call, so we note we are starting purging after the plugin
release is successful.
use correct api call to parse bytes in ConstEstimationMethod

previously used an invalid method from an old branch where
parse_bytes was named differently.
ensure EstimationHook fails when there is insufficient space

Previously this was only logging estimated vs. available and
did not use estimated-size-adjust-by-percent (estimated-size-factor)
to scale the estimate.
Merge branch 'refactor_backup_api' into amsterdam