delegatingresolver: Call ResolveNow in a new goroutine to avoid deadlocks

[arjan-bal]

While handling updates from child resolvers, the delegating resolver locks it's mutex. If the channel returns an error on updating the state, the delegating resolver calls ResolveNow on the other child. This can result in a deadlock if the other child attempts to call cc.UpdateState synchronously while handling the ResolveNow call. This change makes delegatingResovler call ResolveNow in a new goroutine to avoid such deadlocks.

RELEASE NOTES:

• resolver/delegatingresolver: Avoid deadlocks when an http proxy is configured and a resolver calls UpdateState synchronously while handling a ResolveNow call.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.07%. Comparing base (b0d1203) to head (e15d5a2).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #8199   +/-   ##
=======================================
  Coverage   82.06%   82.07%           
=======================================
  Files         410      410           
  Lines       40233    40218   -15     
=======================================
- Hits        33018    33008   -10     
  Misses       5854     5854           
+ Partials     1361     1356    -5     

Files with missing lines	Coverage Δ
.../resolver/delegatingresolver/delegatingresolver.go	80.00% <100.00%> (+3.31%)	⬆️

... and 32 files with indirect coverage changes

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[arjan-bal]

Changes merged into #8195 since all non-test code changes were common.