msg: Add `messenger dump <name>` asok command

msg: Add `messenger dump <name>` asok command

Create admin socket command `messenger dump <name> [filter]
[--tcp-info]` on messenger initialization. `<name>` is the human
readable messenger name (e.g "client", "ms_objecter"). The filter
argument allows partial dumping. `tcp_info` controls inclusion of
TCP_INFO stats. Calling `messenger dump` without a messenger name
returns a list of messengers.

Signed-off-by: Marcel Lauhoff <marcel.lauhoff@clyso.com>