perf(client): optimize with atomic.Int64 for lastSubscribeAt in pkg/varlog.(subscriber)

[ijsong]

What this PR does

Switches the type of pkg/varlog.(subscriber).lastSubscribeAt from
atomic.Value to atomic.Int64 to eliminate heap escapes. The frequent updates
with new time.Time values to lastSubscribeAt previously led to significant
heap allocation due to heap escape. By adopting atomic.Int64, we can prevent
heap escape, thereby reducing heap allocations and enhancing performance.

[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 42.85714% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 49.75%. Comparing base (890508d) to head (a58015b).
Report is 1 commits behind head on main.

❗ Current head a58015b differs from pull request most recent head 617bd9d. Consider uploading reports for the commit 617bd9d to get more accurate results

Files	Patch %	Lines
pkg/varlog/subscribe.go	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #720      +/-   ##
==========================================
+ Coverage   49.67%   49.75%   +0.08%     
==========================================
  Files         235      235              
  Lines       20717    20718       +1     
==========================================
+ Hits        10291    10309      +18     
+ Misses       9783     9773      -10     
+ Partials      643      636       -7     

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[ijsong]

Merge activity

• Mar 5, 4:41 AM EST: @ijsong started a stack merge that includes this pull request via Graphite.
• Mar 5, 4:51 AM EST: Graphite rebased this pull request as part of a merge.
• Mar 5, 4:58 AM EST: @ijsong merged this pull request with Graphite.