A trait for the ability to store client session data. The keys and values are opaque.
Both the keys and values should be treated as highly sensitive data, containing enough key material to break all security of the corresponding session.
put is a mutating operation; this isn't expressed
in the type system to allow implementations freedom in
how to achieve interior mutability.
Mutex is a common
Stores a new
if the value was stored.
Returns the latest value for
if there's no such value.