Merge PR #23985 into master

Merge PR #23985 into master

* refs/pull/23985/head:
	ceph-objectstore-tool: add back pool dne check
	qa/suites/rados/singleton/reg11184: remove old test
	ceph-objectstore-tool: import pg at original epoch
	osd: handle null pg slot on startup
	ceph-objectstore-tool: drop support for ancient export files
	osd: avoid dropping osd_lock when pg osdmaps are not laggy
	qa/standalone/osd/pg-merge.sh: add merge vs pg import test

Reviewed-by: xie xingguo <xie.xingguo@zte.com.cn>
Reviewed-by: Josh Durgin <jdurgin@redhat.com>