MB-10307: bucket I/O priority markup

Change-Id: Ief929d9655ee7cc85b4bbf74475af43eab5915c3
[alk@tut.by: fixed typo & added ticket reference]
Signed-off-by: Aliaksey Kandratsenka <alk@tut.by>
Reviewed-on: http://review.couchbase.org/35677
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>