Fixed MigrateAdminUnitOrgUnitSchema upgradestep, get foreign key constraints names dynamically.
Fixed MigrateAdminUnitOrgUnitSchema upgradestep: dropped unnecessary column change.
Fixed IncreaseColumnLength upgradestep, get foreign key constraints names dynamically.
Fixes IncreaseColumnLength upgradestep for oracle, removed wrong `nullable` parameter.
Dropped not necessary `new_column` attribute in IncreaseColumnLength upgradestep.
Fixed MigrateTaskTable upgradestep.

Migrate data before making the `issuing_orgunit` column required,
because the oracle DB is more strict in this case.
Disable temporarily removing unique constraints when using Oracle as OGDS DB.

Because oracle can handle column renames with unique constraints.
Fixed DB connection setup in the SchemaMigration class.

This allows transaction rollbacks when the upgrade fails.
Made AlterSequenceNumberType upgradestep oracle conform.

Because oracle does not support changing column type's from String to Integer,
we had to implement an oracle specific way with an temporary column.