Use msgbuf_queuelen() instead of accessing wbuf.queue for msgbuf structs.

Use msgbuf_queuelen() instead of accessing wbuf.queue for msgbuf structs.

Also export the msgbuf queue len as a stats member so bgpctl does not need
to dig into the msgbuf structure inside struct peer.
This skips imsg related msgbuf since that will be handled by a imsgbuf
specific function.
OK tb@