Create http.Server with Read and Write timeouts

[stephen-soltesz]

This change fixes #293 and adds similar preventative timeouts to the ServeOnce servers used by NDT5 plain, ws, and wss measurements.

This change adds ReadTimeout and WriteTimeout to the http.Server used by the control channel for all NDT5 and NDT7 servers. The plain server control channel provides it's own timeout logic.

However, for singleserving servers, this change now adds a context timeout in the plainServer.ServeOnce logic and uses the same http.Server timeouts as the control ports for ws and wss servers.

Also fixes #122

For a summary of how http.Server timeouts apply to the connection lifetime, see:
https://blog.cloudflare.com/the-complete-guide-to-golang-net-http-timeouts/

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This change is 

[coveralls]

Pull Request Test Coverage Report for Build 1385

• 36 of 36 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.4%) to 80.862%

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Totals
Change from base Build 1375:	0.4%
Covered Lines:	1707
Relevant Lines:	2111

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💛 - Coveralls

[pboothe]

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