Prevent linking the same instance more than once

[Mark90]

• Before saving instances, ensure that the same subscription instance is not linked more than once. This only counts for the scope of a subscription or subscription instance. In different parts of a subscription's tree the same subscription instance could still be linked
• Bump version to 2.7.0rc3

Closes #726

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 83.40%. Comparing base (ac51829) to head (6fe0fea).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## orchestrator-core-677-refer-same-pb #727 +/- ## ======================================================================= + Coverage 83.36% 83.40% +0.03% ======================================================================= Files 188 188 Lines 9246 9266 +20 Branches 1509 1520 +11 ======================================================================= + Hits 7708 7728 +20 Misses 1273 1273 Partials 265 265 ```

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