poll-loop: Automatically log reason for wakeup when CPU usage spikes.

poll-loop: Automatically log reason for wakeup when CPU usage spikes.

For a long time, the poll-loop module has had the ability to log the reason
for wakeups, which is valuable for debugging excessive use of CPU time.
But I have to ask users to turn up the log level for the module, which
wastes their time and mine.  This commit improves the situation by
automatically logging the reason for a wakeup whenever a process's
estimated CPU usage rises above 50%.  (ovs-vswitchd often uses less than
1% CPU; more than 5% CPU is uncommon.)