gaiad: ExportGenesisFile() incorrectly overwrites genesis

[alessio]

ExportGenesisFile() overwrites all non-gaia related
sections of the genesis.json file as it always
creates a new one from scratch.

Remove cmd/gaia/init.LoadGenesisDoc() in favor of
tendermint's types.GenesisDocFromFile().

Closes: #4066

• Linked to github-issue with discussion and accepted design OR link to spec that describes this work.
• Wrote tests
• Updated relevant documentation (docs/)
• Added a relevant changelog entry: sdkch add [section] [stanza] [message]
• rereviewed Files changed in the github PR explorer

---

For Admin Use:

• Added appropriate labels to PR (ex. wip, ready-for-review, docs)
• Reviewers Assigned
• Squashed all commits, uses message "Merge pull request #XYZ: [title]" (coding standards)

[codecov]

Codecov Report

Merging #4063 into develop will decrease coverage by 0.05%.
The diff coverage is 33.33%.

@@             Coverage Diff             @@
##           develop    #4063      +/-   ##
===========================================
- Coverage    60.01%   59.95%   -0.06%     
===========================================
  Files          212      212              
  Lines        15107    15108       +1     
===========================================
- Hits          9066     9058       -8     
- Misses        5420     5428       +8     
- Partials       621      622       +1

[fedekunze]

ACK