farm: use PRIx64 to print oid

Signed-off-by: Liu Yuan <namei.unix@gmail.com>
Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
cache: reclaim cow objects after sheep restart

Signed-off-by: Liu Yuan <namei.unix@gmail.com>
Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
collie: retry forever to execute requests

This patch add a 'max_retry_count' to exec_req() helper. sheep and sheepfs
still retry MAX_RETRY_COUNT for the rquests. But collie retry forever (UINT32_MAX)

Signed-off-by: Liu Yuan <namei.unix@gmail.com>
Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>