client: pre-encode request before sending to batch-send-loop (#1841)

[ti-chi-bot]

This is an automated cherry-pick of #1841

This PR tries to prevent issues like #1839 by pre-encoding request.

The micro-bench results show that the overhead introduced by this PR is negligible. Besides, we can reduce the mem allocs of AttachContext after this PR.

workload	threads	pre-encode	baseline	diff
pointget	64	83120.35	83940.81	-0.98%
batchget	64	24469.9	24629.87	-0.65%
tblscan	64	41522.92	41565.02	-0.10%
idxscan	64	40363.22	40328.56	0.09%
idxlookup	64	16482.58	16497.08	-0.09%

Summary by CodeRabbit

• Bug Fixes

  • Enhanced error handling for batch request encoding with improved validation of request states.

• Chores

  • Optimized batch request processing through object pooling to reduce memory allocations and improve efficiency.
  • Refined request lifecycle management with better cleanup and state tracking.

• Tests

  • Added comprehensive test coverage for batch encoding, pooling reuse, and concurrent operations to ensure reliability.

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[ti-chi-bot]

@zyguan This PR has conflicts, I have hold it.
Please resolve them or ask others to resolve them, then comment /unhold to remove the hold label.

[coderabbitai]

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[ti-chi-bot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[zyguan]

/retest