Saving websocket RTT samples

[darkk]

That's WIP for #192, reopened per the comment in #232.

I have a question I'm unsure about:

• is it okay to carry start around the way it's implemented? It looks a bit messy to me, but I'm quite okay with that.

WIP is:

• [x] send only the first WSInfo during download test. It breaks the feedback loop and makes queue management easier. Biased RTT estimates are not so interesting in the real time.

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This change is 

[coveralls]

Pull Request Test Coverage Report for Build 1113

• 52 of 232 (22.41%) changed or added relevant lines in 13 files are covered.
• 5 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-6.6%) to 72.66%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
ndt7/upload/sender/sender.go	4	5	80.0%
ndt7/download/sender/sender.go	6	10	60.0%
ndt7/handler/handler.go	2	6	33.33%
ndt7/ping/message/message.go	19	23	82.61%
ndt7/ping/ping.go	0	9	0.0%
ndt7/ping/sender/sender.go	0	31	0.0%
ndt7/ping/receiver/receiver.go	0	60	0.0%
ndt7/ping/mux/mux.go	0	67	0.0%
<!--	Total:	52	232

Files with Coverage Reduction	New Missed Lines	%
ndt7/ping/ping.go	1	0%
ndt7/upload/sender/sender.go	4	67.65%
<!--	Total:	5

Totals
Change from base Build 1111:	-6.6%
Covered Lines:	1576
Relevant Lines:	2169

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💛 - Coveralls