Waffle flags are useful for gradual rollout but that's not possible this
high in the middleware because a lot of the data needed (request.user)
to partition incoming requests is not availabale this high in the
middleware.
Convert this to a WaffleSwitch which will be safer to operate.
Also increase the number of frames in the stack to print per change.
Printing just 1 did not provide enough info because DRF requests wrap
WSGI requests and have a setter proxy. It will be useful to figure out
how they do this in case it's better that what we're doing in the safe
sessions middleware.
Ticket: https://openedx.atlassian.net/browse/ARCHBOM-1718
c1bdf01876