Add sockid to Close

[gfr10598]

tcp-info is already caching the SockID for connections, so should send it with the Close events, instead of requiring clients to cache it too.

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This change is 

[coveralls]

Pull Request Test Coverage Report for Build 918

• 11 of 13 (84.62%) changed or added relevant lines in 4 files are covered.
• 2 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.2%) to 82.748%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
inetdiag/structs.go	2	3	66.67%
saver/saver.go	2	3	66.67%

Files with Coverage Reduction	New Missed Lines	%
inetdiag/structs.go	1	95.69%
saver/saver.go	1	75.3%

Totals
Change from base Build 911:	-0.2%
Covered Lines:	1084
Relevant Lines:	1310

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💛 - Coveralls

[stephen-soltesz]

I don't object to this change. Please enumerate the number of other repos that will want to be updated to use the latest interface. gomod should help pin existing users, but if it's something we can just do quickly to get it out of the way all the better.

Reviewable status: 0 of 1 LGTMs obtained