sheep: recovery without using priority list

Current sheepdog, I/O request is processed by way of the Priority Lists during auto-recovery.
But it might be wasted performance.

This patch omits unnecessary procedure.

[evaluation method and result by Fukuda-san]

[results (response time of vdi write command)]
 /* sheepdog-master */
1st: 2m38.765s
2nd: 2m25.439s
3rd: 2m32.993s
4th: 2m30.355s
5th: 2m27.301s

 average: 2m30.971s

 /*this-patch */
1st: 2m15.284s
2nd: 2m13.842s
3rd: 2m15.636s
4th: 2m14.967s
5th: 2m15.100s

 average: 2m14.966s

[Environment]
sheepdog node: 3
zookeeper node: 3 (same machine with sheepdog node)
copies: 3

[operations]
 /* all sheepdog start */

 * create VDI for test */

 * random data insert in test10G VDI

 /* delete object in one node */

 /* cache clear */

 /* sheepdog start in one node */

   ----->auto-recovery start in backend

 /* vdi write start */

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