Remote get: fix Db name
Command: use chain id as worker id
Db: use log levels
Db union: iterate over results from multiple queries
Support for transaction rollback

When a transaction fails, the chain's direction of execution
is reversed and all executed transaction are rollbacked.

Transaction may have an irreversible flag, which renders the chain
as failed.
Utils::Zfs: add get_confirmed_snapshot_name
Dataset::Transfer: rollback, use zfs utils to get snapshot name
Dataset::Snapshot: rollback
Dataset::Create: rollback
Vps::CopyConfigs: rollback
Vps::CreateRoot: rollback
Vps::Mounts: dummy rollback
Vps::Passwd: dummy rollback
Vps::Restart: dummy rollback
Vps::Start: rollback
Vps::Stop: rollback
Utils::Log: execution direction, remove worker