Bitfinex: Fix bug where checksum calculation is incorrect

[shazbert]

PR Description

Dear community,

This uses #1354 to store a string representation of the amount field to reduce the chance of checksum calculation failure. This is caused by a exponent value because sent on very small amounts which then gets stored as such but then when we strconv.FormatFloat(amountVal, 'f', -1, 64) it then gets turned into a decimal.

Kind regards,

Shazbert.

Type of change

Please delete options that are not relevant and add an x in [] as item is complete.

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] This change requires a documentation update

How has this been tested

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• [ ] go test ./... -race
• [ ] golangci-lint run
• [ ] Test X

Checklist

• [ ] My code follows the style guidelines of this project
• [ ] I have performed a self-review of my own code
• [ ] I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation and regenerated documentation via the documentation tool
• [ ] My changes generate no new warnings
• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] New and existing unit tests pass locally and on Github Actions/AppVeyor with my changes
• [ ] Any dependent changes have been merged and published in downstream modules