Reduce CTL threads priority to about PUSER.

Since in most configurations CTL serves as network service, we found
that this change improves local system interactivity under heavy load.
Priority of main threads is set slightly higher then worker taskqueues
to make them quickly sort incoming requests not creating bottlenecks,
while plenty of worker taskqueues should be less sensitive to latency.

MFC after: 1 week
Sponsored by: iXsystems, Inc.