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

Signed-off-by: Sage Weil <sage@inktank.com>
(cherry picked from commit 6ec2ba5ff426d44f0da75a332818070248464e71)

Conflicts:
	src/common/Makefile.am
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>
(cherry picked from commit f15265447da54a02762729d3b03caf7c146eec1b)
common/WorkQueue: allow io priority to be set for wq

Signed-off-by: Sage Weil <sage@inktank.com>
(cherry picked from commit b3397a3334ff72b232f9363f02a9e9689b2dbc8c)

Conflicts:
	src/common/WorkQueue.cc
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>
(cherry picked from commit 84b3003119eeb8acfb3faacf357e6c6a452950e3)

Conflicts:
	src/osd/OSD.cc