cfq-iosched: Implement tunable group_idle

o Implement a new tunable group_idle, which allows idling on the group
  instead of a cfq queue. Hence one can set slice_idle = 0 and not idle
  on the individual queues but idle on the group. This way on fast storage
  we can get fairness between groups at the same time overall throughput
  improves.

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Acked-by: Jeff Moyer <jmoyer@redhat.com>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>