[node] Add Buffer.poolSize() and correct Buffer.byteLength()

[Flarna]

• Use a meaningful title for the pull request. Include the name of the package modified.
• Test the change in your own code. (Compile and run.)
• Follow the advice from the readme.
• Avoid common mistakes.
• Run npm run lint package-name (or tsc if no tslint.json is present).
• Provide a URL to documentation or source code which provides context for the suggested changes:
  https://nodejs.org/dist/latest-v8.x/docs/api/buffer.html#buffer_class_property_buffer_poolsize
  https://nodejs.org/dist/latest-v8.x/docs/api/buffer.html#buffer_class_method_buffer_bytelength_string_encoding
• Increase the version number in the header if appropriate.
• If you are making substantial changes, consider adding a tslint.json containing { "extends": "dtslint/dt.json" }.

fixes #20399 @vbodnia: would be nice if you can test in your environment
fixes #19930 @guftall: would be nice if you can test in your environment

[dt-bot]

types/node/index.d.ts

to authors (@DefinitelyTyped/DefinitelyTyped @parambirs @robdesideri @tellnes @WilcoBakker @octo-sniffle @smac89 @mwiktorczyk @wwwy3y3 @Tyriar @DeividasBakanas @kjin @alvis Microsoft TypeScript (account can't be detected)). Could you review this PR?
👍 or 👎?

Checklist

• pass the Travis CI test?

[Flarna]

Test failure look unrelated. Besides that travis times out...
@andy-ms @RyanCavanaugh : The list of node PRs lying around with unrelated CI fails grows. Any hints how to improve this?
see #20308, #20185, #20186, #20126, #20068, ...

[typescript-bot]

@Flarna Please fix the failures indicated in the Travis CI log.

[ghost]

@Flarna The way to improve it is for a human to go and actually look at the pull requests... Sorry for the delay.

[Flarna]

@andy-ms Thanks!
Maybe it's possible to improve the Bot to give people submitting a PR the possibility to to mark them as "CI fail is unrelated" and Bot puts them into the human queue.

[ghost]

Either a package should be merged or a human should look at it and help out. There's no third possibility or else we end up with 200 open pull requests.