Cherry-pick #8195 into v1.71.x

[arjan-bal]

Original PR: #8195
Related Issue: #8168

RELEASE NOTES:

• resolver/delegatingresolver: Avoid deadlocks when an http proxy is configured and a resolver calls UpdateState synchronously while handling a ResolveNow call.
• resolver/delegatingresolver: Fix race that may cause panics when an http proxy is configured and the resolver is closing.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.43%. Comparing base (208e03b) to head (720552e).
Report is 1 commits behind head on v1.71.x.

Additional details and impacted files

@@             Coverage Diff             @@
##           v1.71.x    #8202      +/-   ##
===========================================
+ Coverage    82.39%   82.43%   +0.04%     
===========================================
  Files          387      387              
  Lines        38942    38959      +17     
===========================================
+ Hits         32086    32116      +30     
+ Misses        5551     5543       -8     
+ Partials      1305     1300       -5     

Files with missing lines	Coverage Δ
.../resolver/delegatingresolver/delegatingresolver.go	82.22% <100.00%> (+5.53%)	⬆️

... and 24 files with indirect coverage changes

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