public void validateThatSessionIsPrepared(Tenant tenant, long sessionId) {
        Session session = getRemoteSession(tenant, sessionId);
        if ( Session.Status.PREPARE != session.getStatus())
            throw new IllegalArgumentException("Session not prepared: " + sessionId);
    }
Gets the active Session for the given application id.

@return the active session, or null if there is no active session for the given application id.
configserver/src/main/java/com/yahoo/vespa/config/server/ApplicationRepository.java