Add config parameters for requiring in_forward to respond with ack

There are three parameters for ack (two are newly added, one is renamed):

* extend_internal_protocol
  Whether to add an optional hash to the end of the sent data from
  out_forward to in_forward. This is used for requiring ack responses or
  some future functions.

* require_ack_response
  Whether to require in_forward to respond with ack. If true, the
  optional hash contains a chunk id.

* ack_response_timeout
  How many seconds out_forward waits for an ack response in.

To enable ack responses, set both extend_internal_protocol and
require_ack_response to true, and set ack_response_timeout to greater
than zero.