traffic: optimize packet init for better performance

traffic: optimize packet init for better performance

When we run traffic_replay, we are creating millions of Packet objects.
So small change in Packet.__init__ will make big difference.

By initializing packet with converted values without parsing string, the time
cost for 3961148 calls of Packet.__init__ dcrease from 17s to 4s, according
to cProfile.

Signed-off-by: Joe Guo <joeg@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Garming Sam <garming@catalyst.net.nz>