This partially undoes commit 92678b3c

, more specifically it removes the
Store data type and the associated code, since all Luxi arguments are
now lists.

Furthermore, since the qfilter field on Query is complex (it's
actually a tree structure), and we don't support it, turn it into a
plain () type, which always gets encoded as JSNull ('null'), so that
we can remove the optional field handling from Luxi (all fields are
always required).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>