Keep TVar's of individual user caches

It seems that I've invented mechanism to do basic session
management. Snap sessions only offer holding for Text types
and I had to reject snaplet-typed-sessions at this point.
(needs at least version relaxing).

TVar (Map Text (Tvar UserCache)) it is, then.

Signed-off-by: Heikki Salo <heikki.ao.salo@iki.fi>