Use downsample.* constants where possible. Move the downsampling time
ranges into constants and use them as well.

Signed-off-by: Giedrius Statkevičius <giedriuswork@gmail.com>

Fix the column's name and name it UNTIL-DOWN because that is what it
actually shows - time until the next downsampling.

Move out the calculation into a separate function into the compact
package. Ideally we could use the retention policies in this calculation
as well but the `bucket` subcommand knows nothing about them :-(

Signed-off-by: Giedrius Statkevičius <giedriuswork@gmail.com>

Reorder the constants and fix mistakes.

Signed-off-by: Giedrius Statkevičius <giedriuswork@gmail.com>