osd: add assert that we're replica
osd: add assert that we're replica

ar Fred saw a crash where we got into merge_log as a stray, which really
shouldn't ever happen!  See #590.

Signed-off-by: Sage Weil <6dd34506bbd5e58221bbb3e4732d97c91f02277b@newdream.net>

ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph