Fix as-of and since so that they include and exclude time-point respectively

[jonasseglare]

SUMMARY

Implement as-of and since so that they work just like in Datomic.

• Implement as-of so that it includes time-point
• Implement since so that it excludes time-point

Fixes #712

Checks

Bugfix

• [x] Related issues linked using fixes #number
• [x] Integration tests added
• [ ] Architecture Decision Record added if design changes necessary
• [ ] Formatting checked