pkg/xds/v2: add StreamClientMutex.

[chainhelen]

= assignment is not atomic in golang and partial assignments that may
occur probabilistically. ADSClient.StreamClient are both read/written
in two goroutinues, so adding mutex to guarantee concurrent security.

Issues associated with this PR

No.

Sign the CLA

Done.

Solutions

= assignment is not atomic in golang and partial assignments that may
occur probabilistically. ADSClient.StreamClient are both read/written
in two goroutinues, so adding mutex to guarantee concurrent security.

UT result

Probabilistically and theoretically.

Benchmark

Not involve.

Code Style

• Make sure Goimports has run
  Done in goland.
• Show Golint result
  Print nothing.

[chainhelen]

Travis ci failed, but it seems that doesn't involve my modification.

[codecov-io]

Codecov Report

Merging #885 into stable will decrease coverage by <.01%.
The diff coverage is 38.27%.

@@            Coverage Diff             @@
##           stable     #885      +/-   ##
==========================================
- Coverage   55.76%   55.76%   -0.01%     
==========================================
  Files         234      234              
  Lines       23771    23790      +19     
==========================================
+ Hits        13257    13267      +10     
- Misses       9309     9315       +6     
- Partials     1205     1208       +3

Impacted Files	Coverage Δ
pkg/metrics/downstream.go	0% <ø> (ø)	⬆️
pkg/mtls/sds/subscriber.go	56.92% <0%> (ø)	⬆️
pkg/xds/v2/xdsconfig.go	0% <0%> (ø)	⬆️
pkg/router/routers_manager.go	69.69% <0%> (ø)	⬆️
pkg/stream/http/stream.go	10.24% <0%> (ø)	⬆️
pkg/xds/v2/adssubscriber.go	0% <0%> (ø)	⬆️
pkg/network/connection.go	34.13% <0%> (-0.49%)	⬇️
pkg/xds/conv/convertxds.go	25.53% <0%> (-0.04%)	⬇️
pkg/xds/conv/update.go	0% <0%> (ø)	⬆️
pkg/trace/sofa/rpc/span.go	58.33% <100%> (ø)	⬆️
... and 10 more

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[pxzero]

approve

[codecov-io]

Codecov Report

Merging #885 into master will decrease coverage by 0.02%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #885      +/-   ##
==========================================
- Coverage   55.66%   55.63%   -0.03%     
==========================================
  Files         234      234              
  Lines       23794    23802       +8     
==========================================
- Hits        13244    13243       -1     
- Misses       9336     9346      +10     
+ Partials     1214     1213       -1

Impacted Files	Coverage Δ
pkg/xds/v2/adssubscriber.go	0% <0%> (ø)	⬆️
pkg/log/logger.go	55.08% <0%> (-1.61%)	⬇️
pkg/network/connection.go	33.39% <0%> (-0.37%)	⬇️
pkg/module/http2/transport.go	78.73% <0%> (+0.29%)	⬆️

---

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Δ = absolute <relative> (impact), ø = not affected, ? = missing data
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