bench: upgrade tornado to 5.1

[fynntang]

What happened？

There are 1 security vulnerabilities found in tornado 4.3

• CVE-2018-1000518

What did I do？

Upgrade tornado from 4.3 to 5.1 for vulnerability fix

What did you expect to happen？

Ideally, no insecure libs should be used.

The specification of the pull request

PR Specification from OSCS

[mreiferson]

Thanks for the PR, but the benchmark tools are used in a very specific way that makes this low risk. Also, have you tested that it actually works with a major version upgrade of Tornado?