rgw: RGWSI_Notify uses Finisher instead of RGWSI_Finisher

rgw: RGWSI_Notify uses Finisher instead of RGWSI_Finisher

RGWSI_Notify is the only user of RGWSI_Finisher, but doesn't "own" it.
this means RGWSI_Notify can't drain/stop the Finisher on shutdown to
guarantee the lifetime of contexts like C_ReinitWatch that it
schedules there

replace the use of RGWSI_Finisher with direct ownership of Finisher, and
drain/stop it at the end of RGWSI_Notify::shutdown()

Signed-off-by: Casey Bodley <cbodley@redhat.com>
(cherry picked from commit 4aa6ed886be6ca5f1763794e598bc584b303bb64)