REST: Cleanup Tx Broadcasting and Encoding

[alexanderbez]

• Remove encode and broadcast from x/auth client -- it doesn't belong there and broadcasting was already duplicated client/tx.
• Move encode route and command to client/tx package
• Minor cleanup (removing dup/unused code)

closes: #3692

Note, @cosmos/cosmos-ui , this removes the duplicate broadcasting endpoint. However, one endpoint accepted a StdTx and the other accepted raw bytes. I think the former is more desirable and easier for clients. So I updated POST @ /txs to accept a StdTx now.

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• 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.

• Wrote tests

• Updated relevant documentation (docs/)

• Added entries in PENDING.md with issue #

• rereviewed Files changed in the github PR explorer

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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

❗ No coverage uploaded for pull request base (develop@cc938b8). Click here to learn what that means.
The diff coverage is 42.85%.

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[jackzampolin]

Nice! LGTM 👍

[cwgoes]

Duplicate PENDING.md entry I think, otherwise LGTM.