oxenstored: refactor putting response on wire

oxenstored: refactor putting response on wire

Previously, the functions reply_{ack,data,data_or_ack} and input_handle_error
put the response on the wire by invoking Connection.send_{ack,reply,error}.

Instead, these functions now return a value indicating what needs to be put on
the wire, and that action is done by a send_response function called
afterwards.

This refactoring gives us a chance to store the value of the response, useful
for replaying transactions.

Signed-off-by: Jonathan Davies <jonathan.davies@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jon Ludlam <jonathan.ludlam@citrix.com>
Reviewed-by: Euan Harris <euan.harris@citrix.com>
Acked-by: David Scott <dave@recoil.org>