Chain dataset create: validation of DatasetInPool

An exception is raised if DatasetInPool is not valid.
DatasetInPool mountpoint: check if empty, check uniqueness
Transaction RemoteRollback: fix tree and branch params

Branch and tree parameters were set badly. Head was always used,
instead of the real branch/tree where the snapshot is.
Vps.Snapshot show: find any snapshot in the VPS subtree
DatasetInPool mountpoint: forbid two or more slashes in a row
Transaction confirmations: add just_destroy

Just destroy removes the row from the database on transaction success
and does nothing if the transaction fails. It is independent
on the 'confirmed' column.
DatasetInPool destroy: use just_destroy for repeatable tasks
Chains and transaction for per-IP shaper set/unset
VPS offline migration

Note that the migration is not yet safe. It cannot properly recover
from failure without admin intervention. Datasets may remain
on both source and destination nodes, which may render other vpsAdmin
functionality inoperable.

Migration also does not yet use fallbacks and urgent transactions.
It may therefore cause migrated VPS to remain stopped until admin
starts it.