Failure when using Memcached as session handler

[rafaelse]

Since we switched to Memcached for handling sessions, Heartbeat started failing due to requiring a Moodle file that does not exist:

moodle-tool_heartbeat/index.php

Line 151 in 99eaf75

require_once($CFG->libdir . '/classes/session/util.php');

The server responds with error code 500 and FPM produces the following error message:
PHP message: PHP Fatal error: require_once(): Failed opening required '/var/www/html/lib/classes/session/util.php' (include_path='/var/www/html/lib/pear:.:/usr/local/lib/php') in /var/www/html/admin/tool/heartbeat/index.php on line 151

I would like to know if there's any workaround for this problem or any fix planned for it.

Thanks in advance.