bgp: refine the API for connecting to bmp server

We might want a bgp speaker to talk with multiple bmp servers in the
future so I renamed these functions. Note that currently a speaker can
connect to only one bmp server.

I also update the example code on how to make the speaker to connect
to a bmp server.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>