Change calc update

[theflyingcodr]

Fees for change were using the wrong size for the input sig, that has now been adjusted to 71 bytes.

Some of the fee calcs were done in two places, as these round down, they sometimes resulted in the change being 1 satoshi too high which could impact the fee rate. Now moved the actual fee calc to one place to avoid this issue.

[codecov-commenter]

Codecov Report

Merging #33 (57d82cc) into v2 (97fd3b2) will increase coverage by 0.18%.
The diff coverage is 62.50%.

❗ Current head 57d82cc differs from pull request most recent head 5b1443a. Consider uploading reports for the commit 5b1443a to get more accurate results

@@            Coverage Diff             @@
##               v2      #33      +/-   ##
==========================================
+ Coverage   77.99%   78.18%   +0.18%     
==========================================
  Files          18       18              
  Lines        1109     1100       -9     
==========================================
- Hits          865      860       -5     
+ Misses        163      161       -2     
+ Partials       81       79       -2     

Impacted Files	Coverage Δ
txchange.go	78.37% <62.50%> (+2.47%)	⬆️

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