Backups should be compressed

Hi,

We have an influxdb unit with a fairly hefty amount of data:

$ sudo du -hs /var/lib/influxdb
39G /var/lib/influxdb
$

In the charm config, backup_dir is set to "/srv/backups/influxdb", which runs a nightly dump into e.g. /srv/backups/influxdb/2019-10-31/ and keeps the last 3 days worth by default.

Which means we need another ~120GB of free space for backups - actually closer to 160, since new backups are created before the oldest backups are removed.

Just adding a simple tar/gzip step to the backup script would save us a ton of disk space:

38G /srv/backups/influxdb/2019-10-31
8.3G /srv/backups/influxdb/2019-10-31.tgz