[dss] Implicit subscriptions for OIRs are only extended, never shrunk

[Shastick]

Describe the bug

When an OIR is created with parameters that specify an existing implicit subscription,
that subscription's extent will be extended as required to cover the OIR

There is no corresponding mechanism that would shrink the subscription to smallest possible extend when an OIR is deleted, and the implicit subscription it was depending on has other dependent OIRs.

To reproduce

Given t0 < t1 < t2:

• create OIR A from time t0 to t1 and request an implicit subscription which we'll call IS1
• IS1 now goes from t0 to t1
• create an OIR B from t1 to t2 that relies on subscription IS1
• IS1 now goes from t0 to t2
• delete OIR A
• IS1 still goes from t0 to t2

Difference from expected behavior

The expected behavior is that after deleting OIR A, the implicit subscription should have been shrunk to only cover t1 to t2.

Possible solution

Extend the cleanup logic being added with #1056 to shrink a subscription, is possible, when a dependent OIR is being removed.

Notes

Given that the subscription covers all existing OIRs, the only downside of this bug is that a USS may received notifications that are not relevant anymore.

A mitigation for this is for a USS to either let the DSS manage one implicit subscription per OIR or to manage its subscriptions itself.