Add "netx" package to replace fdcache

[stephen-soltesz]

This change adds a new package (tentatively named "netx") that provides a fully managed set of types supporting the net.Listener, net.Conn, and net.Addr interfaces while fully encapsulating the operations needed to support BBR and TCP info operations for "raw", "http" and "https" servers (i.e. ndt5 and ndt7).

This change adds the package. But, due to the change size, it does not yet replace or remove the fdcache package, nor does this change remove the CachingTCPKeepAliveListener.

The changes outside the netx package are minor, changing the name and return type of GetBBRInfo to use primitive types and mirror GetTCPInfo.

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This change is 

[coveralls]

Pull Request Test Coverage Report for Build 1340

• 113 of 117 (96.58%) changed or added relevant lines in 5 files are covered.
• 5 unchanged lines in 3 files lost coverage.
• Overall coverage increased (+0.8%) to 81.111%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
ndt7/measurer/measurer.go	1	5	20.0%

Files with Coverage Reduction	New Missed Lines	%
ndt5/c2s/c2s.go	1	72.32%
ndt7/measurer/measurer.go	1	85.23%
ndt7/upload/sender/sender.go	3	74.29%

Totals
Change from base Build 1338:	0.8%
Covered Lines:	1752
Relevant Lines:	2160

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💛 - Coveralls

[stephen-soltesz]

https://github.com/m-lab/dev-tracker/issues/573

[pboothe]

This looks very very good to me.

This makes the code clearer and better and is just frigging great. Thank you for doing it.

Reviewable status: complete! 1 of 1 approvals obtained (waiting on @pboothe)