Whisk together the wheat flour, rye flour, cornmeal, baking soda, baking powder, salt, and allspice in a medium mixing bowl. Add the molasses, buttermilk, vanilla, and zest, and whisk to combine. Divide the mixture evenly between the prepared cans. Cover the top with a double layer of aluminum foil and tie securely with string. Pour the boiling water into the pot. Carefully place in the oven and bake until the edges of the bread begin to pull away from the sides of the cans, about 1 hour and 15 minutes. Remove the cans from the pot of water, uncover, place on a wire rack, and cool 1 hour before removing bread from the cans. Serve with baked beans or slice, toast, and serve with cream cheese.