xdsclient: release lock before attempting to close underlying transport

[easwars]

This PR fixes a rare deadlock where multiple channels are created and closed to the same target URI (with an xds scheme) at the same time. This deadlock leads to xDS client creation being stalled and therefore everything on the gRPC channel is stalled.

RELEASE NOTES:

• xds: fixes a possible deadlock that happens when both the client application and the xDS management server (responsible for configuring the client) were using the xds:/// scheme in their target URIs

[easwars]

@purnesh42H : FYI

[dfawley]

I would rewrite this a little. The reason to do it without the lock is not because it will "take a while" (because that implies it's purely for performance), but because the things it does could theoretically call into the xdsclient package again and deadlock.

[easwars]

Done. Thanks.

[codecov]

Codecov Report

Attention: Patch coverage is 87.50000% with 1 line in your changes missing coverage. Please review.

Project coverage is 82.03%. Comparing base (eb1adde) to head (3fdb7c5).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
xds/internal/xdsclient/client_refcounted.go	87.50%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8011      +/-   ##
==========================================
- Coverage   82.16%   82.03%   -0.14%     
==========================================
  Files         384      384              
  Lines       38745    38750       +5     
==========================================
- Hits        31836    31788      -48     
- Misses       5598     5636      +38     
- Partials     1311     1326      +15     

Files with missing lines	Coverage Δ
xds/internal/xdsclient/client_refcounted.go	81.81% <87.50%> (-0.24%)	⬇️

... and 22 files with indirect coverage changes