Reduce performance impact of stream stats

[pstavirs]

For 64 byte packets with v1.2.0

• Without stream stats tracking enabled: 490 Mbps
• With stream stats tracking enabled: 385 Mbps

A loss of 100 Mbps (~20%) which I can't explain. A quick look at perf top also doesn't show anything obvious.

The most costly operations seem to be stats_ access/updates within PcapTxThread::sendQueueTransmit() but the whole function itself (except it's children) takes up only about 2% cpu. So it's likely something system side that needs to be studied in greater detail.