FEATURE: Look for optimisations

[RasmusSkytte]

Describe the feature
With the new benchmarks added to diseasystore we should look for ways to optimise the core functions.

Some ideas:

• Use 9999-01-01 instead of NULL as in valid_until to indicate an open record
  This can reduce the operations since we can then rely on "<" and not "< or NULL".
  https://github.com/ssi-dk/diseasystore/tree/refs/heads/feature/simpler-valid_until

• Implement SCDB::create_index() on the generated tables
  https://github.com/ssi-dk/diseasystore/tree/refs/heads/feature/add_indexes

What use will this feature have?
Performance

Who should add the feature?
N/A

Additional context
N/A