fix goroutine leak when tls enabled

[DanielZhangQD]

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]

Deploy 4 TiDB Clusters with TLS enabled and 4 TiDB Clusters with TLS disabled, after 12 hours running with this fix:
The overall profile:

Count	Profile
15699	allocs
0	block
0	cmdline
291	goroutine
15699	heap
0	mutex
0	profile
34	threadcreate
0	trace

The memory usage is less than 60 MB and the heap profile as below:

Fetching profile over HTTP from http://10.233.119.199:6060/debug/pprof/heap
Saved profile in /root/pprof/pprof.tidb-controller-manager.alloc_objects.alloc_space.inuse_objects.inuse_space.022.pb.gz
File: tidb-controller-manager
Type: inuse_space
Time: Aug 5, 2020 at 8:28pm (EDT)
Entering interactive mode (type "help" for commands, "o" for options)
(pprof) top
Showing nodes accounting for 21170kB, 85.44% of 24776.52kB total
Showing top 10 nodes out of 182
      flat  flat%   sum%        cum   cum%
 4440.76kB 17.92% 17.92%  4440.76kB 17.92%  runtime/pprof.writeHeapInternal
 3632.60kB 14.66% 32.58%  3632.60kB 14.66%  reflect.unsafe_NewArray
 3073.03kB 12.40% 44.99%  3073.03kB 12.40%  go.uber.org/zap/zapcore.newCounters
 2560.31kB 10.33% 55.32%  2560.31kB 10.33%  github.com/modern-go/reflect2.newUnsafeStructField
 2049.08kB  8.27% 63.59%  2049.08kB  8.27%  reflect.mapassign
 1536.05kB  6.20% 69.79%  1536.05kB  6.20%  github.com/json-iterator/go.(*Iterator).ReadString
 1301.24kB  5.25% 75.04%  1301.24kB  5.25%  golang.org/x/net/http2.(*ClientConn).frameScratchBuffer
 1024.57kB  4.14% 79.18%  1024.57kB  4.14%  encoding/json.typeFields
 1024.19kB  4.13% 83.31%  1024.19kB  4.13%  github.com/aws/aws-sdk-go/aws/endpoints.init
  528.17kB  2.13% 85.44%   528.17kB  2.13%  k8s.io/apimachinery/pkg/watch.(*Broadcaster).Watch.func1

[codecov-commenter]

Codecov Report

Merging #3081 into master will decrease coverage by 0.01%.
The diff coverage is 87.50%.

@@            Coverage Diff             @@
##           master    #3081      +/-   ##
==========================================
- Coverage   42.34%   42.33%   -0.02%     
==========================================
  Files         156      156              
  Lines       16763    16767       +4     
==========================================
  Hits         7099     7099              
- Misses       9091     9094       +3     
- Partials      573      574       +1     

Flag	Coverage Δ
#unittest	42.33% <87.50%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

[weekface]

LGTM

[ti-srebot]

cherry pick to release-1.1 in PR #3083