CGI: Force geometries to 2D before exporting as GeoJSON.

And skip geometry columns other than the first one.  Some sources
(svk:stolpar, nvr:SPA_Rikstackande, nvr:OSPAR, svk:stationsomraden,
nvr:HELCOM, nvr:SCI_Rikstackande) come as 3D and we don't flatten them
during import.

Dropping the Z (and/or M) component during GeoJSON export avoids
transfering data which will anyway be ignored by OpenLayer.  The
overhead of ST_Force2D() seems negligible in comparison with
ST_AsGeoJSON().  Querying 100000 random features in each table and
measuring yields the following.

Simple query
============

    SELECT convert_to(ST_AsGeoJSON(m.*,geom_column=>'wkb_geometry',pretty_bool=>'f'),'UTF8') AS "GeoJSON"
      FROM postgis."nvr:SCI_Rikstackande" m WHERE m."ogc_fid" = %s

nvr:HELCOM
Time: min=0.000s, max=0.002s, avg=0.000s
Size: min=1.24kiB, max=24.23kiB, avg=9.47kiB

nvr:SCI_Rikstackande
Time: min=0.000s, max=0.663s, avg=0.002s
Size: min=0.84kiB, max=61613.04kiB, avg=259.67kiB
(Yes that's >50MiB, cf. SE0820430 Torne och Kalix älvsystem.)

nvr:SPA_Rikstackande
Time: min=0.000s, max=0.008s, avg=0.001s
Size: min=1.36kiB, max=392.37kiB, avg=35.23kiB

svk:stolpar
Time: min=0.000s, max=0.003s, avg=0.000s
Size: min=0.19kiB, max=0.20kiB, avg=0.19kiB

svk:stationsomraden
Time: min=0.000s, max=0.003s, avg=0.000s
Size: min=0.35kiB, max=1.22kiB, avg=0.56kiB

sks:UtfordAvverk
Time: min=0.000s, max=0.023s, avg=0.001s
Size: min=0.51kiB, max=1117.39kiB, avg=9.75kiB

Advanced query
==============

    WITH feature AS (
      SELECT m."ogc_fid",
             m.…
             ST_Force2D(m."wkb_geometry") AS "wkb_geometry",
             %s AS layer_group,
             %s AS layer
      FROM postgis."nvr:SCI_Rikstackande" m
      WHERE m."ogc_fid" = %s
    )
    SELECT convert_to(ST_AsGeoJSON(feature.*,geom_column=>'wkb_geometry',pretty_bool=>'f'),'UTF8') AS "GeoJSON"
      FROM feature

(We add `layer_group` and `layer` to the later, which adds 48bytes — the
size of `, "layer_group": "layer_group", "layer": "layer"` to the
output.)

nvr:HELCOM
Time: min=0.000s, max=0.004s, avg=0.000s
Size: min=1.21kiB, max=22.62kiB, avg=8.86kiB

nvr:SCI_Rikstackande
Time: min=0.000s, max=0.893s, avg=0.002s
Size: min=0.88kiB, max=57799.83kiB, avg=243.62kiB

nvr:SPA_Rikstackande
Time: min=0.000s, max=0.011s, avg=0.001s
Size: min=1.37kiB, max=367.70kiB, avg=33.13kiB

svk:stolpar
Time: min=0.000s, max=0.010s, avg=0.000s
Size: min=0.23kiB, max=0.23kiB, avg=0.23kiB

svk:stationsomraden
Time: min=0.000s, max=0.004s, avg=0.000s
Size: min=0.38kiB, max=1.21kiB, avg=0.58kiB

sks:UtfordAvverk (already in 2D so skip ST_Force2D() call)
Time: min=0.000s, max=0.085s, avg=0.001s
Size: min=0.55kiB, max=1117.44kiB, avg=9.80kiB

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