ActiveRecord: On reconnection failure, release only failed connetion.

On reconnection failure, all the connection was released.
But, it is better to release only failed connection.
This patch changes not to release all the connection but release
only failed connection.

activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb

@@ -454,7 +454,8 @@ def checkout_and_verify(c)
         end
         c
       rescue
-        disconnect!
+        remove c
+        c.disconnect!
         raise
       end
     end