Reduce vanflow thread context switches

[cliffjansen]

A router under load (such as iperf or h2load) can generate more than 300,000 vanflow records per second. The dedicated vanflow thread will try to publish records as soon as possible, resulting in about 150,000 context switches per second between io worker threads and the vanflow thread.

Since the vanflow output does not require low latency, the thread can be scheduled far less frequently allowing it to do the same amount of work using much less cpu resources.