rgw/multi: Give tasks a reference to RGWDataChangesLog

rgw/multi: Give tasks a reference to RGWDataChangesLog

Also run them in strands. Also `datalog_rados` is a `shared_ptr`,
now. Probably make it intrusive later.

Signed-off-by: Adam C. Emerson <aemerson@redhat.com>
(cherry picked from commit 3c2b587ead6b0cb5acfd84788958dd957d020875)

Conflicts:
	src/rgw/driver/rados/rgw_service.cc
	src/rgw/rgw_sal.cc
 - `#ifdef`s for standalone Rados
	src/rgw/driver/rados/rgw_datalog.cc
 - Periodic re-run of recovery removed in main and pending backport

Signed-off-by: Adam C. Emerson <aemerson@redhat.com>