WIP: Chain Tx Export

[alexanderbez]

• Implement gaiad export-chain command that creates a JSON (can be gzipped) with an export that contains for each block: the height, the proposer, the validators and the txs.

TODO: Determine how/what we really want to do with replay/import? I think this should be just a simple Gaia app creation and replay txs.

closes: #3026

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• Wrote tests

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[codecov]

Codecov Report

Merging #3081 into develop will decrease coverage by 0.41%.
The diff coverage is 0%.

@@             Coverage Diff             @@
##           develop    #3081      +/-   ##
===========================================
- Coverage    52.15%   51.74%   -0.42%     
===========================================
  Files          136      137       +1     
  Lines         9619     9696      +77     
===========================================
  Hits          5017     5017              
- Misses        4265     4342      +77     
  Partials       337      337