common/io_priority: wrap ioprio_set() and gettid()

Signed-off-by: Sage Weil <sage@inktank.com>
common/Thread: allow io priority to be set for a Thread

Ideally, set this before starting the thread.  If you set it after, we
could potentially race with create() itself.

Signed-off-by: Sage Weil <sage@inktank.com>
common/WorkQueue: allow io priority to be set for wq

Signed-off-by: Sage Weil <sage@inktank.com>
osd: allow io priority to be set for the disk_tp

The disk_tp covers scrubbing, pg deletion, and snap trimming

Signed-off-by: Sage Weil <sage@inktank.com>