fix: put conn back

[hzliangbin]

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

Ⅰ. Describe what this PR did

put connection back if no longer need it

Ⅱ. Does this pull request fix one issue?

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

Ⅳ. Describe how to verify it

Ⅴ. Special notes for reviews

[codecov-commenter]

Codecov Report

Merging #164 (bc2a185) into dev (7a213c2) will not change coverage.
The diff coverage is 66.66%.

❗ Current head bc2a185 differs from pull request most recent head 81d72fc. Consider uploading reports for the commit 81d72fc to get more accurate results

@@           Coverage Diff           @@
##              dev     #164   +/-   ##
=======================================
  Coverage   42.29%   42.29%           
=======================================
  Files          73       73           
  Lines        8342     8342           
=======================================
  Hits         3528     3528           
  Misses       4483     4483           
  Partials      331      331           

Impacted Files	Coverage Δ
pkg/filter/dt/filter_http.go	16.34% <66.66%> (ø)
pkg/filter/dt/filter_mysql.go	1.51% <66.66%> (ø)

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[fatelei]

LGTM