Mahara as MNet IDP breaks because of longer session ids

When you use Mahara as the MNet identity provider, it tries to store the user's session id in the "sso_session.sessionid" database column. This column is 40 characters, because we were previously using SHA-1 based sessionids, which are 40 characters. When we switched to SHA-256, the sessionids expanded to 64 characters, which causes it to crash.

To replicate:

1. Set up MNet between Moodle and Mahara, with Mahara as the identity provider.
2. Log in to Mahara.
3. In the sideblock, click on the link to roam over to Moodle

Expected result: You roam over to Moodle
Actual result: It crashes with this error message:

[WAR] d8 (lib/errors.php:796) Failed to get a recordset: postgres8 error: [-1: ERROR: value too long for type character varying(40)] in EXECUTE("INSERT INTO "sso_session" ("userid", "instanceid", "username", "useragent", "token", "confirmtimeout", "expires", "sessionid") VALUES (?, ?, ?, ?, ?, ?, ?, ?)")
Command was: INSERT INTO "sso_session" ("userid", "instanceid", "username", "useragent", "token", "confirmtimeout", "expires", "sessionid") VALUES (?, ?, ?, ?, ?, ?, ?, ?) and values was (userid:1,instanceid:2,username:admin,useragent:3628ed27e34fdc54e674d6a3b4a24c71208a600d,token:9d1b2dcf6adf3ab284b3940113ef76f0513eca93,confirmtimeout:1467678764,expires:1467765149,sessionid:a63792d82ed3538d731018873581817c7214a7ea94e4379316161a0b8c773a7c)
Call stack (most recent first):

    log_message("Failed to get a recordset: postgres8 error: [-1: E...", 8, true, true) at /home/aaronw/www/mahara/htdocs/lib/errors.php:95
    log_warn("Failed to get a recordset: postgres8 error: [-1: E...") at /home/aaronw/www/mahara/htdocs/lib/errors.php:796
    SQLException->__construct("Failed to get a recordset: postgres8 error: [-1: E...") at /home/aaronw/www/mahara/htdocs/lib/dml.php:1088
    insert_record("sso_session", object(stdClass)) at /home/aaronw/www/mahara/htdocs/api/xmlrpc/lib.php:93
    start_jump_session(object(Peer), "2", "") at /home/aaronw/www/mahara/htdocs/auth/xmlrpc/jump.php:53