Fix /become with new ActiveSession code

Fails because the externalId was null, but the column is
declared as NOT NULL.  Make it nullable, and also fix up
the token column to be more clear as session_cookie.

Change-Id: Ie0122d807976330878889c58ed86ceb0f88402a1
Signed-off-by: Shawn O. Pearce <sop@google.com>
1 file changed