Made schemamigrations to the 4.0.x release oracle compatible.

The following fixes and adjustments had to be made for installations which using `Oracle` as OGDS DB:
- Get foreign key constraints names dynamically.
- Dropped  unnecessary or already done column change.
- Fixes `IncreaseColumnLength` upgradestep for oracle, removed wrong `nullable` parameter.
- Dropped not necessary `new_column` attribute in `IncreaseColumnLength` 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.
- Fixed DB connection setup in the SchemaMigration class, this allows transaction rollbacks when the upgrade fails.
 - Made `AlterSequenceNumberType` upgradestep oracle conform.

@deiferni please have a look ...