feat: add graceful node leave

[abarbare]

Hello,

This PR correct the issue 236 from original python-consul repository python-consul/python-consul#236

[codecov]

Codecov Report

Merging #25 (7b1e322) into master (b105755) will decrease coverage by 0.25%.
The diff coverage is 16.66%.

@@            Coverage Diff             @@
##           master      #25      +/-   ##
==========================================
- Coverage   94.45%   94.20%   -0.26%     
==========================================
  Files           6        6              
  Lines        1840     1846       +6     
==========================================
+ Hits         1738     1739       +1     
- Misses        102      107       +5     

Flag	Coverage Δ
unittests	94.20% <16.66%> (-0.26%)	⬇️

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Impacted Files	Coverage Δ
consul/base.py	93.50% <16.66%> (-0.29%)	⬇️

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