Support >255 flows

https://github.com/intel-go/nff-go/blob/master/flow/flow.go#L1161
Casting the uint flowNumber to uint8 limits us to 255 paths/flows. I'd love to support (many) more.

Has anyone tried doing this? 

It looks like perhaps the fix is limited to flow.go? Perhaps simply changing the uint8 -> uint in that file?

Or am I missing something?
