This is incorrect, only reads from the current Raft master are guaranteed to not be stale. In the case of InfluxDB, I think caching is safe because the shard metadata is immutable.
A trait is a type, but the type does not have a size, which is the reason it is not possible to store a trait instance directly in a Vec (or any other place needing a sized type). It is possible to store a pointer to a trait object in a Vec, which would look like Vec<Box<CanineTrait>>.
I am in a similar situation, my answer is to take as much of the Stanford db class over the break as possible, and take the alternate at school. Although the deadline for all the exercises have passed you can still watch the videos and do the exercises / quizzes. http://www.db-class.org