fixbug: Ping should put resource back to the pool

[wybrobin]

ref: https://github.com/cectc/dbpack/issues/

Ⅰ. Describe what this PR did

I found dbpack won`t work as mysql proxy when waiting for a few minutes, and the ping would get the resource from the pool but not put it back to the pool. If the default config capacity is 30 and ping_interval is 20s, the dbpack won`t work after 30*20s=600s for the pool exhausted.

Ⅱ. Does this pull request fix one issue?

no.

Ⅲ. Why don't you add test cases (unit test/integration test)?

I had test it on my local enviroment.

Ⅳ. Describe how to verify it

Run the sample, and wait for more than 10 minutes.

Ⅴ. Special notes for reviews

nothing.

[codecov-commenter]

Codecov Report

Merging #74 (4dc3683) into dev (f798737) will not change coverage.
The diff coverage is n/a.

❗ Current head 4dc3683 differs from pull request most recent head 070f037. Consider uploading reports for the commit 070f037 to get more accurate results

@@           Coverage Diff           @@
##              dev      #74   +/-   ##
=======================================
  Coverage   31.83%   31.83%           
=======================================
  Files          47       47           
  Lines        5776     5776           
=======================================
  Hits         1839     1839           
  Misses       3749     3749           
  Partials      188      188           

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[dk-lockdown]

LGTM

[dk-lockdown]

pls rebase you code with upstream

git remote add upstream  git@github.com:CECTC/dbpack.git
git fetch upstream
git pull --rebase upstream dev
git push -f

then your local code will catch up with dbpack CECTC:dev

[bohehe]

LGTM

[zhu733756]

lgtm