Add /health endpoint for ndt-server

[cristinaleonr]

This PR adds a new /health endpoint for ndt-server, which writes out a 200 status code if the server is healthy (i.e., not in lame duck) and an error code otherwise.

This will be queried by the Heartbeat Service and incorporated into the local health assessment for the instance.

This change is 

[coveralls]

Pull Request Test Coverage Report for Build 1633

• 21 of 21 (100.0%) changed or added relevant lines in 1 file are covered.
• 8 unchanged lines in 4 files lost coverage.
• Overall coverage increased (+0.03%) to 81.643%

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Files with Coverage Reduction	New Missed Lines	%
ndt5/ndt5.go	1	89.5%
ndt5/protocol/protocol.go	2	83.86%
ndt5/web100/web100_linux.go	2	94.44%
ndt5/c2s/c2s.go	3	72.32%
<!--	Total:	8		-->

Totals
Change from base Build 1623:	0.03%
Covered Lines:	1819
Relevant Lines:	2228

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💛 - Coveralls