Apply ownership and viability policies for ontologies managed by scopes and sessions

So far scopes and sessions behave absolutely the same when it comes to handling ontologies added to / removed from them.

The following differentiated policies should be applied:

Scope/Space -> conservative behaviour:

• tries to take ownership of an unversioned ontology only if it is being added anew (i.e. it was not stored in Stanbol earlier).
• when it goes down, it releases all owned ontologies, but does not destroy them.

Session -> more aggressive behaviour:

• tries to take ownership of any unversioned ontologied added to it, no matter what the source.
• when it goes down, it destroys all owned ontologies that were not stored before addition, and releases all the others.

This also gives a whole new meaning to the setUp()/tearDown() pair on ontology collectors.

Ownership can be managed by matching OWL2 versionIRIs with the IDs of ontology collectors (Note: what if the IDs are equal but namespaces differ, e.g. when migrating across Stanbol installations or in a cloud?)