cls/fifo: Journal is flat_set, not multimap

cls/fifo: Journal is flat_set, not multimap

We don't really need the overhead and complexity of a multimap.

Fixes: https://tracker.ceph.com/issues/57562
Signed-off-by: Adam C. Emerson <aemerson@redhat.com>
(cherry picked from commit fcaa45d32d22653c6c8272a7a43af41bf3dc7e88)
Conflicts:
	src/rgw/cls_fifo_legacy.cc
	src/cls/fifo/cls_fifo_types.h
 - Upstream C++20
 - Have to write our own less-than since upstream uses the Spaceship Operator
Fixes: https://tracker.ceph.com/issues/58403
Signed-off-by: Adam C. Emerson <aemerson@redhat.com>