Add-Path Support for IPv6

[czerwonk]

resolves #96

[coveralls]

Pull Request Test Coverage Report for Build 771

• 159 of 205 (77.56%) changed or added relevant lines in 15 files are covered.
• 4 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.3%) to 70.973%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
protocols/bgp/server/update_sender.go	61	62	98.39%
protocols/bgp/packet/update.go	3	4	75.0%
protocols/bgp/server/fsm_address_family.go	16	18	88.89%
protocols/bgp/server/fsm.go	12	16	75.0%
protocols/bgp/packet/bgp.go	4	8	50.0%
protocols/bgp/server/fsm_open_confirm.go	0	5	0.0%
protocols/bgp/server/peer.go	25	33	75.76%
protocols/bgp/server/fsm_open_sent.go	1	22	4.55%

Files with Coverage Reduction	New Missed Lines	%
protocols/bgp/server/update_sender.go	2	87.39%
protocols/bgp/server/fsm_open_sent.go	2	16.67%

Totals
Change from base Build 765:	0.3%
Covered Lines:	3604
Relevant Lines:	5078

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💛 - Coveralls

[taktv6]

LGTM