Use performance.now() where available

[robertodauria]

This PR changes the NDT5 client code to use the monotonic performance.now() instead of Date.now() where available. This fixes a rare bug that happens when the system clock is changed while a measurement is running, which would cause an incorrect client-side throughput to be displayed -- in extreme cases, even a negative value.

(The code is basically the same as in ndt7-js)

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This change is 

[stephen-soltesz]

:lgmt:

Reviewed 1 of 1 files at r1.
Reviewable status: complete! 1 of 1 approvals obtained

[coveralls]

Pull Request Test Coverage Report for Build 1637

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 15 unchanged lines in 4 files lost coverage.
• Overall coverage decreased (-0.7%) to 81.463%

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Files with Coverage Reduction	New Missed Lines	%
ndt5/ndt5.go	2	88.95%
metrics/metrics.go	4	60.0%
ndt5/c2s/c2s.go	4	71.43%
ndt7/upload/sender/sender.go	5	65.91%

Totals
Change from base Build 1634:	-0.7%
Covered Lines:	1815
Relevant Lines:	2228

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💛 - Coveralls