Returns the id of the session to acquire. <p/> Acquiring sessions by ID only is a suitable strategy when sessions are natively managed by Shiro directly. For example, the Servlet specification does not have an API that allows session acquisition by session ID, so the session ID alone is not sufficient for ServletContainer-based SessionManager implementations.
Default implementation of the {@link SessionKey} interface, which allows setting and retrieval of a concrete {@link #getSessionId() sessionId} that the {@code SessionManager} implementation can use to look up a {@code Session} instance.