client is disabled by disable_client.
(may_read_more): New method, extracted from read_from_connection.
	Return false if disabled by disable_client.
(should_be_disabled): New method, extracted from read_from_connection
	and check_idle_callback. Return true if disabled by
	disable_client.
(read_from_connection): Simplified code by using may_read_more and
	should_be_disabled.
(check_idle_callback): Simplified code by using should_be_disabled.
(disable_client): New debug request.