Adjust htools code to new Luxi argument format

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>