Merge PR #37876 into master

* refs/pull/37876/head:
	systemd: cephfs-mirror systemd unit files
	doc, man: man page for `cephfs-mirror` tool
	doc: document mirror daemon internals
	cephfs-mirror: switch to using PeerReplayer class
	cephfs-mirror: cancel ongoing snapshot syncs on dir removal
	cephfs-mirror: display peer snapshot sync stats
	cephfs-mirror: carve out (and implement) mirroring snapshots to peers
	cephfs-mirror: remove `cephfs_mirror_directory_choose_policy` option
	cephfs-mirror: include helper routines to separate source
	cephfs-mirror: remove peer only when peer is tracked
	cephfs-mirror: typedef ceph_mount_info as MountRef shared pointer
	cephfs-mirror: enclose json dump in object section
	cephfs-mirror: note current peer set
	cephfs-mirror: fix option typo and document certain options
	cephfs-mirror: remove unnecessary command line options
	cephfs-mirror: default log level 0/5

Reviewed-by: Patrick Donnelly <pdonnell@redhat.com>