Remove disconnected clients from the pool

Might fix #458
Fixes CartoDB/CartoDB-SQL-API#135

Author: Sandro Santilli

lib/pool.js

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