backing: improve session buffering for runtime information

[alindima]

Where did the idea come from?

#3229 (comment)

What was done?

# 6284 (PR)

What is left to do?

• Cache validator_groups and group_rotation_info got from request_validator_groups call to runtime (note that the now field of the GroupRotationInfo will be different for each relay parent within the session);
• Avoid wrapping caching function errors in RuntimeApiError. Instead, the return value of those functions should be Result<..., Error>;
• Move the cached attributes from PerSessionCache to the general-purpose RuntimeInfo (src: backing: improve session buffering for runtime information #6284 (review)).