R4R: Don't lock keys database on read only ops

[alessio]

Leverage Tendermint's new dbm.NewGoLevelDBWithOpts() call to avoid locking a keys database on read-only operations. This allows piping gaiacli commands's stdouts and stdins.

Ref. #966

• Linked to github-issue with discussion and accepted design OR link to spec that describes this work.
• Wrote tests
• Updated relevant documentation (docs/)
• Added entries in PENDING.md with issue #
• 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 #2544 into develop will decrease coverage by 0.01%.
The diff coverage is 0%.

@@             Coverage Diff             @@
##           develop    #2544      +/-   ##
===========================================
- Coverage    60.28%   60.27%   -0.02%     
===========================================
  Files          150      150              
  Lines         8589     8591       +2     
===========================================
  Hits          5178     5178              
- Misses        3067     3069       +2     
  Partials       344      344

[alessio]

/cc @jackzampolin

[cwgoes]

Do we want to test this with Voyager (cc @faboweb)?

[alexanderbez]

Some suggestions and a comment, but otherwise, utACK 👍

[alessio]

Let's merge this instead: #2489