md full sync: skip sections that have no items (returning 404)

Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
md full sync: put results in the right queue

Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
md partial sync: handle 404 locking a log

Signed-off-by: Josh Durgin <josh.durgin@inktank.com>