endpointsharding: shuffle endpoint order before updating children

[dfawley]

See b/412468630, and esp. comment 46. Essentially, if the connection order is not randomized then the first address gets hammered with all the client's initial requests when traffic is bursty. If multiple clients are all using the same order and many start up around the same time, then the first address can see that problem magnified even further. Randomizing at least spreads the load between the clients, even if it still results in one address getting hammered at startup.

The diffs are much smaller than it shows. Basically, the existing e.s._test.go was moved to ..._ext_test.go and the only new code is in ..._test.go. _ext_test.go is completely unchanged from the old _test.go. Weightedtarget's tests needed changes because they assumed the order in which subchannels were created matched the order in which endpoints were provided.

RELEASE NOTES:

• round_robin: randomize the order in which addresses are connected to in order to spread out initial RPC load between clients.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.38%. Comparing base (64a6b62) to head (e05f10c).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8438      +/-   ##
==========================================
+ Coverage   82.34%   82.38%   +0.04%     
==========================================
  Files         414      414              
  Lines       40411    40421      +10     
==========================================
+ Hits        33276    33302      +26     
+ Misses       5771     5762       -9     
+ Partials     1364     1357       -7     

Files with missing lines	Coverage Δ
balancer/endpointsharding/endpointsharding.go	93.71% <100.00%> (+0.38%)	⬆️

... and 21 files with indirect coverage changes

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