state: prepend environment UUID to unit document IDs

This is a step towards multi-environment state servers.
state: refactoring of env UUID upgrades tests

The tests for the upgrade steps that add environment UUID to the unit
and service collections now share common checker functions.
state: refactoring of env UUID upgrades

The the upgrade steps that add environment UUID to the unit and
service collections now share common generic implementation.
Renamed service env UUID upgrade step to be slightly clearer
state: check env UUID upgrade steps with concrete doc types

The previous generic approach is extended to also load migrated docs
into a document structs, ensuring that entity specific fields are
preserved during the migration.
state: removed Service.Unit()

State.Unit() is equivalent so Service.Unit() was redundant.