rename 'timestamp' meta to 'timestamp-upload'

we also need other timestamps, so this is for clarity
implement 'timestamp-download'
implement purge --inactivity (using 'timestamp-download') for purging objects that have long not seen any download
add support for items with old metadata

fix the metadata so the current code will be happy with it.

python -m bepasty.cli.object purge --age 9999999 ../config.py '*'