zfs: lower priority of zio_write_issue threads by four

The difference of one was insignificant because zio_write_issue threads
ended up on the same run queues as other zio threads.
See sys/priority.h and sys/runq.h for more details.

Add a comment describing FreeBSD priority considerations and restore
the illumos variant of the code for comparison.

Obtained from: Panzura
MFC after: 2 weeks
Sponsored by: Panzura