Enhancement/fee quotes

[theflyingcodr]

This rejigs the Fees a bit and encapsulates the maps etc behind a (hopefully) friendlier API.

It has been extensively documented with godoc comments but it brings in:

• cache quotes from multiple miners
• or use FeeQuote if you only work with one miner
• expiry time for quotes
• easy easy way to add / update a quote/s
• easy way to change quote/s expiry time
• if using FeeQuotes, convenience methods to get fees and update fees

[github-actions]

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[codecov-commenter]

Codecov Report

Merging #18 (8ec856a) into master (d161baa) will increase coverage by 0.40%.
The diff coverage is 89.28%.

❗ Current head 8ec856a differs from pull request most recent head 6285416. Consider uploading reports for the commit 6285416 to get more accurate results

@@            Coverage Diff             @@
##           master      #18      +/-   ##
==========================================
+ Coverage   78.18%   78.58%   +0.40%     
==========================================
  Files          18       18              
  Lines        1100     1158      +58     
==========================================
+ Hits          860      910      +50     
- Misses        161      165       +4     
- Partials       79       83       +4     

Impacted Files	Coverage Δ
bscript/address.go	85.96% <ø> (ø)
fees.go	91.39% <89.04%> (-8.61%)	⬇️
txchange.go	78.37% <90.90%> (ø)

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[jadwahab]

LGTM, just need to sort the merge conflicts with lastest master and we should be good to go! @theflyingcodr