generic-pool should use PromiseLike, not Promise

[wrboyce]

• 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: <bluebird 3.0 definifion is not assignable to ES6 Promises #11027>
• Increase the version number in the header if appropriate.
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[typescript-bot]

@wrboyce Thank you for submitting this PR!

🔔 @jerray - please review this PR in the next few days. Be sure to explicitly select Approve or Request Changes in the GitHub UI so I know what's going on.

If no reviewer appears after a week, a DefinitelyTyped maintainer will review the PR instead.

[typescript-bot]

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[jerray]

PromiseLike is better. thank you.