Add a building is null condition to the landcover layer

Adds a `building IS NULL` to the landcover SQL.

The motivation is that this allows substantial performance increases with the right indexes and that `landcover` is not buildings, as well as fixing some subtle styling oddities.

The performance gain is at least +3.9% over f5ef47835e435cde9353f65b95953f45818092c2 with the greatest gains shown at high zooms with +5.5% at z18 and +2.9% at z13. A partial index `WHERE building IS NULL` as described in https://github.com/gravitystorm/openstreetmap-carto/issues/207#issuecomment-25689455 was present in all cases.

Buildings are drawn on top of this layer, so there isn't a rendering impact.

### Details on why this doesn't impact rendering ###
The old SQL filtering condition is

```sql
WHERE "landuse" IS NOT NULL
  OR "leisure" IS NOT NULL
  OR "aeroway" IN ('apron','aerodrome')
  OR "amenity" IN ('parking','university','college','school','hospital','kindergarten','grave_yard')
  OR "military" IN ('barracks','danger_area')
  OR "natural" IN ('field','beach','desert','heath','mud','grassland','wood','sand','scrub')
  OR "power" IN ('station','sub_station','generator')
  OR "tourism" IN ('attraction','camp_site','caravan_site','picnic_site','zoo')
  OR "highway" IN ('services','rest_area')
```

This pull just adds `AND building IS NULL` to it.

Additionally, the `CASE` statement in the columns effectively restricts `leisure` to `'swimming_pool', 'playground', 'park', 'recreation_ground', 'common', 'garden', 'golf_course'` and `landuse` to 

```
'quarry', 'vineyard', 'orchard', 'cemetery', 'grave_yard', 'residential', 'garages', 'field',
 'meadow', 'grass', 'allotments', 'forest', 'farmyard', 'farm', 'farmland', 'recreation_ground', 
'conservation', 'village_green', 'retail', 'industrial', 'railway', 'commercial', 'brownfield',
'landfill', 'greenfield', 'construction'
```

I was initially concerned that adding `building IS NULL` would impact rendering of features like `amenity=school`, but this is not so.

It turns out that if there is a building tag, the building is rendered on top of the landuse layer, obscuring it, giving it only a very minor rendering effect around the edges due to anti-aliasing.
As an example, see these two buildings, one `building=yes` `amenity=school` and one a normal `building=yes`

![89786](https://f.cloud.github.com/assets/1190866/1279939/9fda4574-2f44-11e3-9e90-5bf9342cacbd.png)![89785](https://f.cloud.github.com/assets/1190866/1279940/a1a028a6-2f44-11e3-83fe-2ee2b695be89.png)

There is a difference but it is **extremely** subtle. The building fill is also very subtly different.

The same thing will apply to any building - the building polygon will be drawn on top of the landcover polygon