Remove client from pool on connection end

Might fix brianc#458
Definitely fixes CartoDB/CartoDB-SQL-API#135

Author: Sandro Santilli

Diff for: lib/pool.js

@@ -33,6 +33,11 @@ var pools = {
             pool.destroy(client);
           });
 
+          // Remove client from pool on connection end
+          client.on('end', function() {
+            pool.destroy(client);
+          });
+
           return cb(null, client);
         });
       },