Add missing @Override annotations

[basil]

Using the@Override annotation consistently is desirable for two reasons:

• It produces a compiler warning if the annotated method doesn't actually override anything (e.g., if there is a misspelling).
• It improves readability by making it explicitly clear that methods are overridden.

[oleg-nenashev]

Will likely need codestyle checker if we want to enforce it in the future.

We may merge it in 24 hours if there is no negative feedback. Please see the merge process documentation for more information about the merge process