oxenstored: refactor request processing

oxenstored: refactor request processing

Encapsulate the request in a record that is passed from do_input to
process_packet and input_handle_error.

This will be helpful when keeping track of the requests made as part of a
transaction.

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>