crimson/osd: keep ClientRequest alive during `seastar::repeat()`

crimson/osd: keep ClientRequest alive during `seastar::repeat()`

before this change, `this` is wrapped by a smart pointer which is
in turn captured by multiple continuations. the continuation with the
longest life cycle is the one passed to `seastar::repeat()`. where
the underlying `repeater` captures the continuation as its member variable.
and `repeater` is not destroyed before `seastar::repeat()` returns.

so, we only need to capture `opref` in the continuation passed to
`seastar::repeat()`.

Signed-off-by: Kefu Chai <kchai@redhat.com>