fix all bits broken by viper API changes

[alessio]

github.com/spf13/viper's recent releases introduced a semantic
change in some public API such as viper.IsSet(), which have
broken some of our flags checks. Instead of checking whether
users have changed a flag's default value we should rely on such
defaults and adjust runtime behaviour accordingly. In order to do
so, it's important that we pick sane defaults for all our flags.

The --pruning flag and configuration option now allow for a
fake custom strategy. When users elect custom, then the
pruning-{keep,snapshot}-every options are interpreted and
parsed; else they're ignored.
Zero is pruning-{keep,snapshot}-every default value. When
users choose to set a custom pruning strategy they are
signalling that they want more fine-grainted control, therefore
it's legitimate to expect them to know what they are doing and
enter valid values for both options.

Closes: #5964

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For contributor use:

• Targeted PR against correct branch (see CONTRIBUTING.md)
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• Code follows the module structure standards.
• Wrote unit and integration tests
• Updated relevant documentation (docs/) or specification (x/<module>/spec/)
• Added relevant godoc comments.
• Added a relevant changelog entry to the Unreleased section in CHANGELOG.md
• Re-reviewed Files changed in the Github PR explorer

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For admin use:

• Added appropriate labels to PR (ex. WIP, R4R, docs, etc)
• Reviewers assigned
• Squashed all commits, uses message "Merge pull request #XYZ: [title]" (coding standards)

[codecov]

Codecov Report

Merging #5982 into master will decrease coverage by 0.02%.
The diff coverage is 88.46%.

@@            Coverage Diff             @@
##           master    #5982      +/-   ##
==========================================
- Coverage   58.27%   58.25%   -0.03%     
==========================================
  Files         396      396              
  Lines       23595    23603       +8     
==========================================
- Hits        13750    13749       -1     
- Misses       8883     8893      +10     
+ Partials      962      961       -1     

[alexanderbez]

utACK. Thanks @alessio.

[jgimeno]

Good one!! :D