[Issue 493] close cnxPool when closing a Client

[XuanYang-cn]

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(If this PR fixes a github issue, please add Fixes #<xyz>.)

Fixes #493

(or if this PR is one task of a github issue, please add Master Issue: #<xyz> to link to the master issue.)

Master Issue: #

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[wolfstudy]

LGTM +1

[wolfstudy]

@XuanYang-cn Thanks your work for this, Is it convenient to explain this pull request comment?

[XuanYang-cn]

The linked issue #493 explains this PR pretty well，shortly, this PR will release TCP connections after we close the client and prevent goroutine from leaking