sheep: update a comment in request_in_recovery()

The comment is outdated since we don't have a farm in sheep now.

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

This moves all the check of recovering objects from
prepare_schedule_oid() to oid_in_recovery(), and makes the code easier
to read.

This also fixes a problem that sheep doesn't recover an object when
 - sys->cinfo.disable_recovery is true,
 - rinfo->state is RW_PREPARE_LIST, and
 - oid_in_recovery() is called against the object
and fixes an error of tests/functional/010, which happens in rare
cases.

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