mem: Allow overriding the default buffer pool.

[vanja-p]

The default buffer pool only has a only a few tiers, which hurts performance in some applications. In #8770, it was decided that instead of adding more tiers to the default pool, we should allow changing the default pool.

RELEASE NOTES:

• mem: Allow changing the default buffer pool

[vanja-p]

I didn't know if you internal.SetDefaultBufferPool should be moved from internal/internal.go to internal/experimental.go so I kept it as is for now.

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.29%. Comparing base (bccbb10) to head (faa1732).
⚠️ Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
experimental/experimental.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8806      +/-   ##
==========================================
- Coverage   83.30%   83.29%   -0.01%     
==========================================
  Files         418      417       -1     
  Lines       33004    32980      -24     
==========================================
- Hits        27493    27470      -23     
+ Misses       4102     4099       -3     
- Partials     1409     1411       +2     

Files with missing lines	Coverage Δ
internal/internal.go	60.00% <ø> (ø)
internal/leakcheck/leakcheck.go	90.04% <100.00%> (ø)
mem/buffer_pool.go	96.61% <100.00%> (ø)
experimental/experimental.go	25.00% <0.00%> (-8.34%)	⬇️

... and 29 files with indirect coverage changes

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[vanja-p]

I can't figure out how to get the PR validation to pass. My initial PR title didn't start with "mem:", but now it does.

[eshitachandwani]

@vanja-p can you check if the test failing is related to the changes or not?
https://github.com/grpc/grpc-go/actions/runs/20763701462/job/59624817820?pr=8806

[vanja-p]

@vanja-p can you check if the test failing is related to the changes or not? https://github.com/grpc/grpc-go/actions/runs/20763701462/job/59624817820?pr=8806

@eshitachandwani, looks like it's not related to my change. Rerunning them passed, and also, my 2 new lines don't have any coverage.