Optimize lwout_geojson output

[Algunenano]

Related to ​​https://trac.osgeo.org/postgis/ticket/4543

After #4543 is applied, around half the time spent in the output of ST_AsGeoJSON of a big polygon (>100k vertices) is spent in sprintf calls, mainly calculating the length of the strings (which we already know). It should be possible to remove all this time by replacing it a memcpy calls or writting the doubles directly in the buffer (instead of writting them in an auxiliar variable and then into the buffer).

[Raúl Marín <git@…>]

In 892fcde0/git:

Geojson: Print doubles directly on the output buffer

Closes #4615
Closes ​https://github.com/postgis/postgis/pull/537