give NAT the ability to have rules with an IdleTimeout #40

our current fork of NetCore is now capable of generating OpenFlow rules which timeout after they have been idle for a certain number of seconds (up to int16 or 65535 seconds, a bit over 18 hours)

to use this functionality from NetCore, change (for example):

Action(aportaction)

to

let metadata = [NetCore_Types.IdleTimeout (OpenFlow0x01_Core.ExpiresAfter n)] in
ActionWithMeta(aportaction, metadata)

where n is the number of seconds. (see https://github.com/adferguson/frenetic/blob/netcore/lib/NetCore_Types.mli#L80 for the various rule metadata options, of which there is only one, IdleTimeout, for now :-)

Tim -- any chance you can expose this up to the FlowLog syntax with appropriate handling in Partial_Eval.ml ?

thanks!