Issue 494, 650, 659, 663, 665, 669: Switch to GNAT Community 2021 and add preparatory work for session support

[treiher]

Description

Closes #494
Closes #650 (only adds preparatory work; the full implementation will be part of #292)
Closes #659
Closes #663
Closes #665
Closes #669

Review Checklist

Reviewer: @senier

Implementation

• Does this code change accomplish what it is supposed to do?
• Can this solution be simplified?
• Does this change add unwanted compile-time or run-time dependencies?
• Was a framework, API, library, service used that should not be used?
• Was a framework, API, library, service not used that could improve the solution?
• Is the code at the right abstraction level?
• Is the code modular enough?
• Would you have solved the problem in a different way that is substantially better in terms of the code’s maintainability, readability, performance, security?
• Does similar functionality already exist in the codebase? If so, why isn’t this functionality reused?
• Are there any best practices, design patterns or language-specific patterns that could substantially improve this code?

Logic Errors and Bugs

• Can you think of any use case in which the code does not behave as intended?
• Can you think of any inputs or external events that could break the code?

Error Handling and Logging

• Is error handling done the correct way?
• Should any logging or debugging information be added or removed?
• Are error messages user-friendly?
• Are there enough log events and are they written in a way that allows for easy debugging?
• Is the style of error / logging messages consistent?

Dependencies

• If this change requires updates outside of the code, like updating the documentation, configuration, readme files, was this done?
• Might this change have any ramifications for other parts of the system, or backward compatibility?

Security and Data Privacy

• Does this code open the software up for security vulnerabilities?
• Does this code change reveal some secret information like keys, passwords, or usernames (e.g., checked-in credentials)?

Performance

• Do you think this code change will impact system performance in a negative way?
• Do you see any potential to improve the performance of the code?

Usability

• Is the proposed solution well designed from a usability perspective?
• Is the API well documented?
• Is the API/UI intuitive to use?

Testing

• Does it have enough automated tests (unit/integration/system tests)?
• Are there some test cases, input or edge cases that should be tested in addition?

Readability

• Was the code easy to understand?
• Can the readability of the code be improved by smaller methods?
• Can the readability of the code be improved by different function/method or variable names?
• Is the code located in the right file/folder/package?
• Do you think certain methods should be restructured to have a more intuitive control flow?
• Is the data flow understandable?
• Are there redundant comments?
• Could some comments convey the message better?
• Would additional comments make the code more understandable?
• Could some comments be removed by making the code itself more readable?
• Is there any commented out code?

Experts Opinion

• Do you think a specific expert, like a security expert or a usability expert, should look over the code before it can be committed?
• Will this code change impact different teams?
• Should they have a say on the change as well?