sheep, dog: add speed throttling of auto-recovery

Current auto recovery to consume as much as possible the resources of sheepdog node.

So, this patch intended to allow the speed throttling of auto-recovery.
By speed throttling, reduce the resource consumption at auto recovery.

Add new options to sheep and dog commands.
Its options are "interval" and "object processing number".

see examples follows.

ex) sheep -R max=5,interval=1000 /var/lib/sheepdog
    dog node recovery set-throttle 5 1000
    dog node recovery get-throttle

Signed-off-by: Yasuhito Fukuda <fukuda.yasuhito@po.ntts.co.jp>
Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>