Ceph

The ceph tell mds.X cache dump and ceph tell mds.X ops commands have a useful --path argument that directs the daemon to save the dump of the output to a file local the daemon. This has several advantages:

• We don't need to construct the JSON output in memory, which may be many gigabytes.
• Streaming writes to a local file is significantly faster than sending the data over the ceph messenger.
• The command spends as little time as possible holding relevant locks.

However, only some commands support this and we could make it generic to the admin socket interface. So, add a generic --output-file argument to achieve this.

The main concern with this is security (telling the daemon to write to arbitrary files) but this is gated by the session caps which require "allow_all" and that's typically only valid for the client.admin.