v1.17: Change DEFAULT_TPU_CONNECTION_POOL_SIZE to 1 (backport of #1014)

[mergify]

Problem

DEFAULT_TPU_CONNECTION_POOL_SIZE was set to 4 to maximize the performance when we do local performance testing with bench-tps. With the server side QOS restrictions per stake node and unstaked node, the benefit of more connections is less obvious while having more connections increase the load on the server especially during leader change.
This only change the default to 1. Validators still can use existing --tpu-connection-pool-size to override the default value.

Summary of Changes

Change DEFAULT_TPU_CONNECTION_POOL_SIZE to 1.

Fixes #

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This is an automatic backport of pull request #1014 done by [Mergify](https://mergify.com).

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.6%. Comparing base (579f540) to head (6eb1a18).
Report is 1 commits behind head on v1.17.

Additional details and impacted files

@@            Coverage Diff            @@
##            v1.17    #1042     +/-   ##
=========================================
- Coverage    81.6%    81.6%   -0.1%     
=========================================
  Files         806      806             
  Lines      219332   219332             
=========================================
- Hits       179111   179103      -8     
- Misses      40221    40229      +8     

[t-nelson]

i'm gonna bow out of bp approval since i approved master