fix goroutine leak when tls enabled (#3081)

[ti-srebot]

cherry-pick #3081 to release-1.1

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What problem does this PR solve?

Fix #3079

What is changed and how does it work?

Disable keepalive for TLS connections, more detail refer to elastic/cloud-on-k8s#854 (comment)

Check List

Tests

• Unit test
• E2E test
• Stability test
• Manual test (add detailed scripts or steps below)
  • Deploy, upgrade, scaling TiDB Cluster with TLS enabled (find one bug with upgrading TiKV cannot upgrade when TLS is enabled #3078)
  • Deploy, upgrade, scaling TiDB Cluster with TLS disabled

Code changes

• Has Go code change

Related changes

• Need to cherry-pick to the release branch

Does this PR introduce a user-facing change?:

Fix goroutine leak when TLS enabled

[DanielZhangQD]

LGTM

[DanielZhangQD]

/merge

[ti-srebot]

/run-all-tests

[codecov-commenter]

Codecov Report

❗ No coverage uploaded for pull request base (release-1.1@b22f180). Click here to learn what that means.
The diff coverage is n/a.

@@              Coverage Diff               @@
##             release-1.1    #3083   +/-   ##
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  Coverage               ?   44.52%           
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  Files                  ?      148           
  Lines                  ?    15804           
  Branches               ?        0           
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  Hits                   ?     7037           
  Misses                 ?     8203           
  Partials               ?      564           

Flag	Coverage Δ
#unittest	44.52% <0.00%> (?)

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