Adapt to Protocol v11

[overcat]

Resolve #197 #199

[theaeolianmachine]

Thanks for getting started on this! Let me know if you have any questions about the changes here.

[codecov]

Codecov Report

Merging #198 into master will decrease coverage by 2.14%.
The diff coverage is 47.22%.

@@            Coverage Diff             @@
##           master     #198      +/-   ##
==========================================
- Coverage   91.07%   88.92%   -2.15%     
==========================================
  Files          13       13              
  Lines        1501     1553      +52     
==========================================
+ Hits         1367     1381      +14     
- Misses        134      172      +38

Impacted Files	Coverage Δ
stellar_base/builder.py	90.47% <20%> (-5.43%)	⬇️
stellar_base/operation.py	94.2% <54.38%> (-5.8%)	⬇️

[codecov]

Codecov Report

Merging #198 into master will decrease coverage by 0.53%.
The diff coverage is 81.94%.

@@            Coverage Diff             @@
##           master     #198      +/-   ##
==========================================
- Coverage   91.07%   90.53%   -0.54%     
==========================================
  Files          13       13              
  Lines        1501     1553      +52     
==========================================
+ Hits         1367     1406      +39     
- Misses        134      147      +13

Impacted Files	Coverage Δ
stellar_base/builder.py	90.47% <20%> (-5.43%)	⬇️
stellar_base/operation.py	99.77% <98.24%> (-0.23%)	⬇️
stellar_base/horizon.py	56.4% <0%> (ø)	⬆️