In d1e1ace3186d ("Allow Flow Routines to be cancellable (#40)"), the
payload was passed to another goroutine and erased by the next packet
to be received if the goroutine did not process it fast enough. Make
a copy before passing it to the goroutine to fix that.
* adds dataframe link decoding
* can map NetFlow/IPFIX fields and bytes sections from sFlow/packets to any field inside the protobuf
* add CLI argument for loading a mapping yaml file
* Source and destination mac addresses are fed from `NFV9_FIELD_xx_yyy_MAC`
Fixes a bug where the samples were missing either source or destination Mac.
This is supposed to be linked to the sampling direction.
