Apply patch to enable the use of the @ character in SQL queries

[enekid]

This issue documents a bug related to the use of the @ character in the SQL queries sent to mapnik. It has being patched in the future 3.1 mapnik release, but not in v3.0.15 (the version used currently in this fork), because it has breaking changes.

Mapnik 3.0.X has a bug with the @ character in the SQL queries. Whenever it finds a @ it treats it as a render-time variable. Therefore the use of the @ character corrupts the SQL query (eg. email addresses: tesla@baddogs.com).

#1136 could be fixed applying the patch, but at the same time, all uses of render-time variables in windshaft should be surrounded by ! characters.

[Algunenano]

Initial branch with the commits modified to be compatible with our 3.0.15 build in https://github.com/Algunenano/mapnik/tree/v3.0.15-carto_postgis_quoting

Pending reviewing Windshaft/Windshaft-cartodb for render-time-variables usages in queries.

[Algunenano]

I have it working locally with the branch above:

I haven't seen any usage of render-time variables but I have to do a deeper search.

[Algunenano]

I haven't found any usage of this kind of variables in Windshaft. @enekid did you see anything while you were investigating the issue?

[enekid]

Nothing. I was warned by @rafatower about this. Maybe he can guide us.

[Algunenano]

I've only seen it used in the 1000x branch (https://github.com/CartoDB/Windshaft-cartodb/blob/1000x-tt/lib/cartodb/utils/substitution-tokens.js) so I'm guessing we should be ok.

[Algunenano]

Merging. To be tested in next Mapnik release.