make packet stats interval configurable.

[JustinAzoff]

The packet stats interval is currently hardcoded to 5000. The time interval itself is configurable via -timeinterval but this is only checked every 5000 packets. At low packet rates the interval will be missed. At high packet rates it may be more efficient to have it even higher.

I can possibly make this self tuning but that may not be worth the effort. It may also be possible to just use a ticket to flip a boolean every timeinterval