Fix potential infinite loop in CDB_QueryStatements

This fix was already present at one point in cartodb/lib/sql
(where the code was copied from) but in a different branch than
the one the code was initially copied from.

The fix depends on plpython language which becomes a new dependency.

test/CDB_QueryStatementsTest_expect

@@ -7,3 +7,13 @@
 4|4|SELECT 5
 5|1|INSER INTO "my''""t" values ('''','""'';;')
 5|2|SELECT $qu;oted$ hi $qu;oted$
+6|1|SELECT
+1
+6|2|SELECT
+2
+7|1|/a
+7|2|b
+7|3|c
+7|4|d
+8|1|SELECT $quoted$ hi
+$quoted$