cache: move batching to demux so that ringBuffer stores single item

[apullo777]

Please read https://github.com/etcd-io/etcd/blob/main/CONTRIBUTING.md#contribution-flow.
#20477 follow-up to move batching into demux to further simplify ringbuffer interface
@serathius @MadhavJivrajani

[k8s-ci-robot]

Hi @apullo777. Thanks for your PR.

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[serathius]

/ok-to-test

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: apullo777, serathius

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 62.49%. Comparing base (c218423) to head (09a384a).
⚠️ Report is 8 commits behind head on main.

Additional details and impacted files

see 147 files with indirect coverage changes

@@            Coverage Diff             @@
##             main   #20502      +/-   ##
==========================================
- Coverage   69.15%   62.49%   -6.67%     
==========================================
  Files         418      405      -13     
  Lines       34696    33590    -1106     
==========================================
- Hits        23995    20991    -3004     
- Misses       9300    11025    +1725     
- Partials     1401     1574     +173     

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