Looks up a session from the underlying data store based on the specified session key.
@param key the session key to use to look up the target session.
@return the session identified by {@code sessionId}.
@throws UnknownSessionException if there is no session identified by {@code sessionId}.
Looks up a session from the underlying data store based on the specified session key.
@param key the session key to use to look up the target session. @return the session identified by {@code sessionId}. @throws UnknownSessionException if there is no session identified by {@code sessionId}.