nclient6: Use cenkalti/backoff/v4 library to replace backoff algorith…

[heinz-boehmer]

…m in #296.

[codecov[bot]]

Codecov Report

Merging #439 (1a30de3) into master (b259fbf) will decrease coverage by 0.00%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #439      +/-   ##
==========================================
- Coverage   67.97%   67.97%   -0.01%     
==========================================
  Files          90       90              
  Lines        3766     3759       -7     
==========================================
- Hits         2560     2555       -5     
+ Misses       1037     1035       -2     
  Partials      169      169              

Flag	Coverage Δ
integtests	∅ <ø> (∅)
unittests	67.97% <100.00%> (-0.01%)	:arrow_down:

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Impacted Files	Coverage Δ
dhcpv6/nclient6/client.go	45.67% <100.00%> (-1.07%)	:arrow_down:

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[pmazzini]

I am not a huge fan of introducing a new dependency but I also don't have a strong opinion here. So I will let others chime in.

[pmazzini]

Fixes #308