Water layer river mapping bug fixes (#1182)

This PR is a bugfix for the `water` layer.

* `waterway=stream`, `waterway=river`, `waterway=canal`, `waterway=ditch`, and `waterway=drain` are all linear features, not area features.  Thus, these objects are being unnecessarily mapped into the `osm_water_polygon` polygon table, and this PR removes these unneeded mappings.
* The combination `natural=water` + `water=river` is the most popular tagging for river areas.  However, the current mapping causes rivers tagged in this way to be rendered in the vector tiles as a `lake`.  This PR adds a check for the `water=river` tag and tags both variants of river areas as `class=river`.

`natural=water` + `water=river` river mapping:
![image](https://user-images.githubusercontent.com/3254090/129825551-388491de-549e-4843-80cc-01dba358c360.png)

`waterway=riverbank` river mapping:
![image](https://user-images.githubusercontent.com/3254090/129825618-4239eae7-a2bc-4a82-9931-fda2c02c2b40.png)


Lake mapping for a `natural=water` (with no other tagging):
![image](https://user-images.githubusercontent.com/3254090/129825554-9394b3d3-988a-4e92-a9f8-b198c695ec37.png)

layers/water/mapping.yaml

@@ -75,6 +75,9 @@ tables:
     - name: waterway
       key: waterway
       type: string
+    - name: water
+      key: water
+      type: string
     - name: is_intermittent
       key: intermittent
       type: bool
@@ -95,11 +98,8 @@ tables:
       - bay
       - spring
       waterway:
-      - river
       - riverbank
-      - stream
-      - canal
-      - drain
-      - ditch
       - dock
+      water:
+      - river
     type: polygon