Copy constructor for result set and entity

Summary

These changes support a better handling of the ResultSet and Entity objects when their owners are to be deleted (e.g. in clients which delete transansaction due to end of the scope of a function while they return an entity or the resultset of said transaction)

These changes are needed for caosdb-julialib!12 (merged)

The server is just being updated for version-compatibility reasons: caosdb-server!31 (merged)

Focus

• Refactoring of the ResultSet and Entity class' copy constructors (amd a general clean-up of the Entity constructor)
• Additional Transaction::ReleaseResultSet and ResultSet::release_at methods which release the unique_ptr from their owning objects.

Test Environment

• CPP Integration tests in caosdb-cppinttest!15 (merged)
• Julia integration tests in

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• [ ] Up-to-date CHANGELOG.md
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include/caosdb/entity.h

@@ -327,6 +327,7 @@ public:
   // friend class Parent;
   // friend class Property;
 
+private:
   inline Messages() : RepeatedPtrFieldWrapper(){};
 };