Retrieves the Session object from the underlying EIS identified by <tt>sessionId</tt> by delegating to
the {@link #doReadSession(java.io.Serializable)} method. If {@code null} is returned from that method, an
{@link UnknownSessionException} will be thrown.
@param sessionId the id of the session to retrieve from the EIS.
@return the session identified by <tt>sessionId</tt> in the EIS.
@throws UnknownSessionException if the id specified does not correspond to any session in the EIS.
Retrieves the Session object from the underlying EIS identified by <tt>sessionId</tt> by delegating to the {@link #doReadSession(java.io.Serializable)} method. If {@code null} is returned from that method, an {@link UnknownSessionException} will be thrown.
@param sessionId the id of the session to retrieve from the EIS. @return the session identified by <tt>sessionId</tt> in the EIS. @throws UnknownSessionException if the id specified does not correspond to any session in the EIS.