Nuke sessions from the database when users logout

Summary:
@tomo ran into an issue where he had some non-SSL-only cookie or whatever, so
"Logout" had no apparent effect. Make sure "Logout" really works by destroying
the session.

I originally kept the sessions around to be able to debug session stuff, but we
have a fairly good session log now and no reprorted session bugs except for all
the cookie stuff. It's also slightly more secure to actually destroy sessions,
since it means "logout" breaks any cookies that attackers somehow stole (e.g.,
by reading your requests off a public wifi network).

Test Plan: Commented out the cookie clear and logged out. I was logged out and
given a useful error message about clearing my cookies.

Reviewers: jungejason, nh, tuomaspelkonen, aran

Reviewed By: aran

CC: tomo, aran, epriestley

Differential Revision: 911