Retry on connection failure when multiple servers are available.

[robertodauria]

Closes #58

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This change is 

[bassosimone]

🐳

[stephen-soltesz]

- I updated the coveralls.io settings for ndt7-client-go to publish a summary to the PR (that was previously disabled) and when I manually went to check on this build to see whether the new changes were covered it seems that the top level files (ndt7.go) are not covered -- I'm not sure why... Doesn't have to be this PR but it would be worthwhile verifying that the primary library files are also included in the coverage reports/ unit testing - e.g. https://coveralls.io/builds/48452574

Reviewable status: complete! 1 of 1 approvals obtained

[robertodauria]

It seems top-level files aren't listed under the "Tree" tab, but they are in the "List" tab:

I was a bit surprised by that 100% since that's not what vscode tells me when enabling the coverage overlay. I think other packages (or main) are covering the few remaining paths, and the -coverpkg flag to go test allows a package's tests to cover code from a different package (basically treating the whole library as a single unit). Nice.