perf(spanner): Use arenas to speed up queries that fetch many rows

[andrewro-google]

Tested by running multiple_rows_cpu_benchmark --project=${GOOGLE_CLOUD_PROJECT} --instance=${GOOGLE_CLOUD_CPP_SPANNER_TEST_INSTANCE_ID} --table-size=1000000 --maximum-channels=8 --maximum-threads=16 --iteration-duration=5 --samples=60 --experiment=select-string --use-only-clients=true --query-size=100000 (larger --query-size than in the README). The data is noisy but seems like a ~20% win on CpuTime. I don't expect this to help much with queries that return few results. For the use case I care about (fetching a whole table), we were limited by CPU when using all the threads on a machine.

We still have a copy when generating Row in PartialResultSetSource::NextRow() but we'd need a new API to avoid that.

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This change is 

[andrewro-google]

@scotthart could you take a look / do the gcbrun incantations

[scotthart]

/gcbrun

[codecov]

Codecov Report

❌ Patch coverage is 99.67105% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 93.04%. Comparing base (612245c) to head (5df9ce8).
⚠️ Report is 55 commits behind head on main.

Files with missing lines	Patch %	Lines
...loud/spanner/internal/partial_result_set_resume.cc	87.50%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #15441   +/-   ##
=======================================
  Coverage   93.04%   93.04%           
=======================================
  Files        2403     2403           
  Lines      219553   219644   +91     
=======================================
+ Hits       204273   204368   +95     
+ Misses      15280    15276    -4     

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[andrewro-google]

I updated the code to fix the clang tidy warning

[scotthart]

/gcbrun

[diegomarquezp]

We should merge main, which brings a fix for the failing MacOS test: #15448

[andrewro-google]

Done, thanks

[scotthart]

@scotthart reviewed 11 of 12 files at r1, 1 of 1 files at r2, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @andrewro-google)

[scotthart]

/gcbrun

[scotthart]

/gcbrun