ndt7: start refactoring to reduce the number of channels

[bassosimone]

As documented in #238, we would like to reduce the pipeline
structure and the dependency on channels. We have a reasonably
clear idea of what we want to do.

Let us do this piecemeal.

In this first diff, I'm changing the measurer to be a
struct rather than just a func. By doing that, I'm
creating the opportunity for storing the measurements
collected by the measurer into the measurer itself rather
than publishing them downstream to a channel.

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This change is 

[pboothe]

Reviewed 3 of 3 files at r1.
Reviewable status: complete! 1 of 1 approvals obtained (waiting on @pboothe)

[coveralls]

Pull Request Test Coverage Report for Build 1189

• 19 of 19 (100.0%) changed or added relevant lines in 3 files are covered.
• 3 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.02%) to 79.942%

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Files with Coverage Reduction	New Missed Lines	%
ndt7/upload/sender/sender.go	3	67.65%

Totals
Change from base Build 1187:	-0.02%
Covered Lines:	1654
Relevant Lines:	2069

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💛 - Coveralls