Proposed changes

• This PR adds vm.opdebug flag which enables collecting and printing the execution time of the opcodes when node stops. This flag is for debugging purpose when measuring execution time and deciding computation cost of the opcodes.
• Originally, the collecting of the execution time is done by uncommenting the debug code in blockchain/vm/interpreter.go. This PR removes the uncommented debug code and add a similar debug code which is run if the vm.opdebug flag is set.

Types of changes

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• Bugfix
• New feature or enhancement
• Others

Checklist

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• I have read the CONTRIBUTING GUIDELINES doc
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• Lint and unit tests pass locally with my changes ($ make test)
• I have added tests that prove my fix is effective or that my feature works
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• Any dependent changes have been merged and published in downstream modules

Related issues

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Further comments

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