Session TTL set arbitrarily high

This came up in the dev chat with Jonathan Champ.

In the session live, session.gc_maxlifetime is (seemingly arbitrarily) set to 4 days:
https://github.com/moodle/moodle/blob/master/lib/classes/session/manager.php#L256

This means a few things:

• You (probably) store sessions for way longer than needed
• You can't set session timeout to move that 4 days (not that you are likely too...)
• You can't tell when using sessions like memcache(d) if you have enough memory allotted.
  • Normally you could watch for evictions, and if you have them, you don't have enough memory, but if you have 2 hr sessions, you have 94 hours of bad sessions that may be evicted.

Looking back at the session rework in MDL-31501, the previous code set session.gc_maxlifetime to sessiontimeout, but when the code was updated, it was just changed to 4 days.

I understand the theory that we want Moodle session cleanup to take precedence. Personally I would set it to "sessiontimeout + 1hr" or something like that.