Async : temporary fix

MultipartReply:
Change type in the parse fn of the dependent modules
add List.rev to change the way we parse the list (fix the unitary test)

PortDescReply :
- clean of sizeof fun
- add to_string fun

Flow:
- delelete the length records (now calculate)
- add to_string fun
- fix sizeof fun
- fix the offset of the Cstruct send to the marshal function

Misc:
move ofp_aggregate_stats_reply inside the module

add unitary test for:
- multipart reply
- flow
- portdesc