bgp: add neighbor_update
bgp: add neighbor_update

added neighbor_update method to BGPSpeaker class.
Currently only MED value can be changed.

Signed-off-by: Hiroshi Yokoi <e59dd578f413d3787f7f28c9bb6236efb1779349@po.ntts.co.jp>
Signed-off-by: FUJITA Tomonori <93dac1fe9c4b2a3957982200319981492ad4976e@lab.ntt.co.jp>

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