RGW: set down_flag early on sync processor threads during shutdown

RGW: set down_flag early on sync processor threads during shutdown

Set down_flag on all sync-related threads (meta sync, data sync,
and log trimmer) before draining or joining them in RGWRados::finalize().

This early flagging ensures that any in-progress data sync operations
can detect shutdown sooner and stop issuing new async processor requests,
reducing the chance of deadlock during realm reload.

Fixes: https://tracker.ceph.com/issues/66100
Signed-off-by: Oguzhan Ozmen <oozmen@bloomberg.net>