Remove an unnecessary Arc from the Session

[elliottt]

ObjectStores internally is an Arc to an RwLock, so the outer Arc present in Session is not necessary. This PR removes that unnecessary indirection, as ObjectStore already derives Clone.

[elliottt]

LGTM. Rust idiomatic q: I notice that SecretStores has HashMap<String, SecretStore>, is one placement of the Arc preferable to the other?

I think it depends on what the desired external use of the value is: putting the Arc inside means that the value is clonable, while leaving it out means that the user of the type has more control over data indirection.

[acw]

LGTM. Rust idiomatic q: I notice that SecretStores has HashMap<String, SecretStore>, is one placement of the Arc preferable to the other?

I don't know if I've detected a general law, and I don't think clippy has never directed me in one direction or another. I can say that my general rule is:

• If the data in the object can be freely cloned without any sort of problem, then don't use an Arc.
• If some/all of the data in the object needs to be shared across concurrent threads/tasks and I care about how parts of it are synchronized, then I put the lock in the object. So, if I really think the right lock is RwLock and always will be RwLock.
• I'm increasingly of the opinion that if I don't care exactly how things are locked, but I do think stuff will be shared, then I make sure the object doesn't implement Clone and have my callers work it out.

[athomason]

Thanks!