ReplicatedBackend: don't queue Context outside of ObjectStore with obc

ReplicatedBackend: don't queue Context outside of ObjectStore with obc

We only flush the ObjectStore callbacks, not everything else.  Thus,
there isn't a guarrantee that the obc held by pull_complete_info will
be cleaned up before the Flush callback is triggered.  Instead, just
defer clearing the pull state until the callback (it'll be cleaned up
during the interval change) and remove the ObjectContext from
pull_complete_info.

Introduced: 68defc2b0561414711d4dd0a76bc5d0f46f8a3f8
Fixes: http://tracker.ceph.com/issues/18927
Signed-off-by: Samuel Just <sjust@redhat.com>
(cherry picked from commit 51eee55c475a3d931844831e040ed3d66ee59af4)