crimson/net: Message::conn needs to be a foreign_ptr

crimson/net: Message::conn needs to be a foreign_ptr

There are two main consequences of this:
1. Messages can't be default copy constructed in crimson.  MMonCommand
   seems to be the only user, and we simply add a copy constructor that
   duplicates data portions of the message.
2. We can't casually copy-construct the conn into other structures.
   The main user here is watch/notify.  We use copy() explicitely
   to populate the object_context structures and avoid passing
   ConnectionFRef by value.

Signed-off-by: Samuel Just <sjust@redhat.com>