[Tile-serving] [osm2pgsql-dev/osm2pgsql] Transform all estimated extents to EPSG:3857 (PR #2498)

[Nakaner]

Nakaner left a comment (osm2pgsql-dev/osm2pgsql#2498)

> Haven't look at it in detail yet, but ST_SRID(way) is definitely wrong, because it hardcodes the geometry column.

Oh, sorry, that `way` should not have made it into the pull request. I pushed a new commit.

> What happens if table is empty?

If the table is empty, `NULL` are returned by both the old and the new query. However, if geometries have no SRID set, the `ST_Transform` throws an error: "ST_Transform: Input geometry has unknown (0) SRID". I pushed a new commit fixing this.

Old implementation (for empty tables, it does not matter if the geometry column has an SRID set or not):

```sql
CREATE TABLE test (osm_id BIGINT NOT NULL, geom geometry(Point, 3857));
ANALYZE test;
SELECT ST_XMin(e), ST_YMin(e), ST_XMax(e), ST_YMax(e) FROM ST_EstimatedExtent('public', 'test', 'geom') AS e; -- NULLs returned
SELECT ST_XMin(e), ST_YMin(e), ST_XMax(e), ST_YMax(e)
  FROM ST_Transform(
    ST_SetSRID(
      ST_EstimatedExtent('public', 'test', 'geom'), 
      (SELECT ST_SRID(geom) FROM public.test LIMIT 1)
    ),
  3857) e; -- NULLs returned
```

If the SQL query returns `NULL`s, an invalid extent will be returned (and logged as "Source table empty, nothing to do.").

By looking on the code a second time, I found that an unset SRID will cause an error. I fixed this in a new commit.

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