Problem: evm transfer is not efficient

[yihuang]

Description

Solution:

• override the Transfer callback in block context to do transfer directly, instead of doing burning and minting.

This makes crypto-org-chain/cosmos-sdk#239 unnecessary, will revert later.

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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

Attention: Patch coverage is 36.00000% with 16 lines in your changes are missing coverage. Please review.

Project coverage is 63.23%. Comparing base (be00d15) to head (b00ab53).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #448      +/-   ##
===========================================
- Coverage    63.29%   63.23%   -0.07%     
===========================================
  Files          125      125              
  Lines        12075    12097      +22     
===========================================
+ Hits          7643     7649       +6     
- Misses        3889     3900      +11     
- Partials       543      548       +5     

Files	Coverage Δ
x/evm/keeper/state_transition.go	72.10% <100.00%> (ø)
x/evm/keeper/statedb.go	84.92% <0.00%> (-1.37%)	⬇️
x/evm/statedb/statedb.go	78.24% <36.36%> (-2.71%)	⬇️