BMP: Use OPEN messages stored in BGP

BMP: Use OPEN messages stored in BGP

The BMP protocol needs OPEN messages of established BGP sessions to
construct appropriate Peer Up messages. Instead of saving them internally
we use OPEN messages stored in BGP instances. This allows BMP instances
to be restarted or enabled later.

Because of this change, we can simplify BMP data structures. No need to
keep track of BGP sessions when we are not started. We have to iterate
over all (established) BGP sessions when the BMP session is established.
This is just a scaffolding now, but some kind of iteration would be
necessary anyway.

Also, the commit cleans up handling of msg/msg_length arguments to be
body/body_length consistently in both rx/tx and peer_up/peer_down calls.