Consider manifests for pruning based on count

If the `riak_cs` application env `max_scheduled_delete_manifests` is set
to an integer (default is `unlimited`), manifests will not only be
pruned based on `scheduled_delete_time`, but based on the number of
`scheduled_delete` manifests there are. If there are more than
`max_scheduled_delete_manifests`, they will be sorted by time (more
recent first), and then cut-off at `max_scheduled_delete_manifests`.
This means that in cases where a key is overwritten frequently, we have
the option of pruning manifests more frequently than just the leeway
time.