feat: merge _http_agent.js from 7.2.1

[fengmk2]

patch from https://github.com/nodejs/node/blob/v7.2.1/lib/_http_agent.js

What's different from original http.Agent?

• keepAlive=true by default
• Disable Nagle's algorithm: socket.setNoDelay(true)
• Add free socket timeout: avoid long time inactivity socket leak in the free-sockets queue.
• Add active socket timeout: avoid long time inactivity socket leak in the active-sockets queue.

[mention-bot]

@fengmk2, thanks for your PR! By analyzing the history of the files in this pull request, we identified @pangorgo, @pmalouin and @alexpenev-s to be potential reviewers.

[codecov-io]

Current coverage is 92.82% (diff: 89.23%)

Merging #45 into master will decrease coverage by 3.28%

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  Files             5          4     -1   
  Lines           283        251    -32   
  Methods          25          0    -25   
  Messages          0          0          
  Branches         59          0    -59   
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- Hits            272        233    -39   
- Misses           11         18     +7   
  Partials          0          0          

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[fengmk2]

3.0.0