Remove disconnected clients from the pool

Might fix brianc#458
Fixes CartoDB/CartoDB-SQL-API#135
Avoids loop between pool.destroy and client.end

Author: Sandro Santilli

Diff for: lib/pool.js

@@ -33,10 +33,20 @@ var pools = {
             pool.destroy(client);
           });
 
+          // Remove connection from pool on disconnect
+          client.on('end', function(e) {
+            // Do not enter infinite loop between pool.destroy
+            // and client 'end' event...
+            if ( ! client._destroying ) {
+              pool.destroy(client);
+            }
+          });
+
           return cb(null, client);
         });
       },
       destroy: function(client) {
+        client._destroying = true;
         client.end();
       }
     });