bgpd: recalc peer's sort after changing confed peers

bgpd: recalc peer's sort after changing confed peers

Currently, when AS number of an existing BGP neighbor is added in a BGP
confederation, AS_CONFED_SEQUENCE segment attribute will be missing in
prefixes advertised to the neighbor. Also, receiving prefixes from the
neighbor will be withdrawn because of "Malformed AS path from A.B.C.D".

    neighbor 10.100.200.3 remote-as 123
    bgp confederation identifier 65001
    bgp confederation peers 123

With this change, update peer's sort after adding or removing its AS
number in a BGP confederation.

Signed-off-by: Alexander Chernavin <achernavin@netgate.com>