[SCB-774] fix warn log caused by ReactiveExecutor

[yhs0092]

Follow this checklist to help us incorporate your contribution quickly and easily:

• Make sure there is a JIRA issue filed for the change (usually before you start working on it). Trivial changes like typos do not require a JIRA issue. Your pull request should address just this issue, without pulling in other changes.
• Each commit in the pull request should have a meaningful subject line and body.
• Format the pull request title like [SCB-XXX] Fixes bug in ApproximateQuantiles, where you replace SCB-XXX with the appropriate JIRA issue.
• Write a pull request description that is detailed enough to understand what the pull request does, how, and why.
• Run mvn clean install to make sure basic checks pass. A more thorough check will be performed on your pull request automatically.
• If this contribution is large, please file an Apache Individual Contributor License Agreement.

---

See details in SCB-774

Let the ReactiveExecutor implement java.io.Closeable so that ProducerProviderManager does not print warn log.

[coveralls]

Coverage increased (+0.02%) to 86.717% when pulling 052b5a3 on yhs0092:fix_ReactiveExecutor_causing_warn_log into f7cd7c2 on apache:master.