timeval: Only log poll intervals longer than 50 ms.

timeval: Only log poll intervals longer than 50 ms.

When poll interval-based logging was introduced a long time, we were
actively interested in looking at almost every long poll interval.  But
these days, with OVS working rather well, with pretty good latency, most
of the messages are red herrings that bother some administrators and
provoke false reports.  So this commit suppresses all but the most
egregious long poll intervals that may in fact be worth looking at.

NIC-366.