hcoop-all-db-backup: update backup script

 * Use /srv/backup/database instead of /var/backups
 * Retain 7 days of backups instead of only 2
 * Use pigz for faster compression of mysql backups
 * Use --single-transaction to avoid locks during mysql backups
 * Use native postgres dump format instead of sql dumps for postgres
 * Tidy up commands used to list databases to backup
 * Change postgres dir name in preparation for postgres-10 backups