mon: add monitor RocksDB backup and restore

Implements an opt-in backup mechanism for the monitor using
rocksdb::BackupEngine. Backups run on a schedule when
mon_backup_interval is set, or are triggered manually via
`ceph tell mon.* backup`. Cleanup keeps the last N, hourly,
and daily snapshots, with a free-space guard. Off by default.

Restore is offline: stop the mon and run
  ceph-mon --restore-backup <dir> --yes-i-really-mean-it
optionally with --backup-version (BackupEngine logical version,
as shown by --list-backups). The mon keyring is stashed alongside
the RocksDB backup so a wiped mon_data is recovered end-to-end,
and kv_backend is stamped back when missing.

Co-authored-by: Daniel Poelzleithner <poelzleithner@b1-systems.de>
Signed-off-by: Matthew N. Heler <matthew.heler@hotmail.com>