xds/internal/cdsbalancer: Fix race between processing of security configuration and creation of SubConns

[zasweq]

Fixes #6787.

RELEASE NOTES:

• xds/internal/cdsbalancer: Fix race between processing of security configuration and creation of SubConns

[codecov]

Codecov Report

Merging #6789 (e072275) into master (8645f95) will increase coverage by 0.16%.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6789      +/-   ##
==========================================
+ Coverage   83.24%   83.41%   +0.16%     
==========================================
  Files         285      285              
  Lines       30950    30959       +9     
==========================================
+ Hits        25765    25825      +60     
+ Misses       4099     4056      -43     
+ Partials     1086     1078       -8     

Files	Coverage Δ
xds/internal/balancer/cdsbalancer/cdsbalancer.go	80.64% <100.00%> (+0.40%)	⬆️

... and 14 files with indirect coverage changes

[dfawley]

Looks like we store a pointer in the address's attributes, so this is still racy when the data is changed while the xdsHI is being read.

Note that the handshakeinfo itself has a lock when accessing its methods. Is that sufficient? I guess not given that the updates happen in 3 steps, but if we combined those into one operation, is that enough to fix everything?

[zasweq]

Discussed offline to do atomic read/write to pointer so Conn can get new handshake info if reconnected, implemented in this PR: #6796.