rgw: move RGWReshardBucket lock to its own separate class

rgw: move RGWReshardBucket lock to its own separate class

There are other processes beyond resharding that would need to take a
bucket reshard lock (e.g., correcting bucet resharding flags in event
of crash, tools to remove bucket shard information from earlier
versions of ceph). Pulling this logic outside of RGWReshardBucket
allows this code to be re-used.

Signed-off-by: J. Eric Ivancich <ivancich@redhat.com>
(cherry picked from commit 18ab99cd542cc253d62719a18a9d13e7a312eb15)