rpc: implement internal debug_ API namespace functions

[hanchon]

Ref: #71 (This PR is 1/3 of the issue)

Description

This PR is the first one to start implementing the _debug namespace.
The _debug namespace has 3 parts:

• Internal debug: Profiling functions. This PR.
• Logger configuration: Allows the user the change the logger settings, for example the log level. This needs to be implemented in another PR.
• Trace calls: This calls return blockchain information (Blocks and Transactions). This needs to be implemented to support etherscan.

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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. (This was internally discussed with @fedekunze)
• Code follows the module structure standards.
• [] Wrote unit and integration tests
  (Note: we based the code on the go-etherum codebase, it test some functionality from the runtime lib, but not the API endpoint. For example: https://github.com/ethereum/go-ethereum/blob/master/metrics/runtime_test.go, we can add these tests if they are needed)
• 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)

[fedekunze]

Looks great! Minor stylistic comments only

[fedekunze]

ACK