perf(client): use transmitResultPool to reduce heap allocs

[ijsong]

WIP

[ijsong]

• perf(client): use transmitResultPool to reduce heap allocs #722 👈
• perf(client): prealloc pkg/varlog.(*transmitter).transmitQueue #721
• perf(client): optimize with atomic.Int64 for lastSubscribeAt in pkg/varlog.(subscriber) #720
• perf(client): reuse snpb.SubscribeResponse in RPC handler #719
• perf(benchmark): use grpc.SharedBufferPool for grpc.DialOption #718
• ci: bump go.uber.org/mock/mockgen from v0.3.0 to v0.4.0 #733 : 1 other dependent PR (#732 )
• main

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[codecov]

Codecov Report

Attention: Patch coverage is 0% with 23 lines in your changes are missing coverage. Please review.

Project coverage is 49.63%. Comparing base (2bd0a62) to head (94fad3a).

Files	Patch %	Lines
pkg/varlog/subscribe.go	0.00%	23 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #722      +/-   ##
==========================================
- Coverage   49.72%   49.63%   -0.10%     
==========================================
  Files         235      235              
  Lines       20725    20731       +6     
==========================================
- Hits        10305    10289      -16     
- Misses       9780     9797      +17     
- Partials      640      645       +5     

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