[WIP] Set the consistent-index as the raft's Applied index

[ahrtr]

This is the first and the most important step to resolve #20187.

I will do a bootstrap performance comparison later.

[k8s-ci-robot]

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[codecov]

Codecov Report

❌ Patch coverage is 62.00000% with 19 lines in your changes missing coverage. Please review.
✅ Project coverage is 68.39%. Comparing base (10838fc) to head (95c4c2d).
⚠️ Report is 70 commits behind head on main.

Files with missing lines	Patch %	Lines
server/etcdserver/bootstrap.go	66.66%	10 Missing and 2 partials ⚠️
server/etcdserver/api/membership/storev2.go	16.66%	5 Missing ⚠️
server/etcdserver/cindex/cindex.go	75.00%	2 Missing ⚠️

Additional details and impacted files

Files with missing lines	Coverage Δ
server/etcdserver/cindex/cindex.go	89.65% <75.00%> (-2.80%)	⬇️
server/etcdserver/api/membership/storev2.go	58.82% <16.66%> (+5.88%)	⬆️
server/etcdserver/bootstrap.go	64.81% <66.66%> (+0.07%)	⬆️

... and 30 files with indirect coverage changes

@@            Coverage Diff             @@
##             main   #21091      +/-   ##
==========================================
- Coverage   68.44%   68.39%   -0.05%     
==========================================
  Files         429      429              
  Lines       35281    35321      +40     
==========================================
+ Hits        24147    24157      +10     
- Misses       9729     9754      +25     
- Partials     1405     1410       +5     

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[k8s-ci-robot]

@ahrtr: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-etcd-grpcproxy-integration-amd64	95c4c2d	link	true	/test pull-etcd-grpcproxy-integration-amd64
pull-etcd-grpcproxy-integration-arm64	95c4c2d	link	true	/test pull-etcd-grpcproxy-integration-arm64
pull-etcd-coverage-report	95c4c2d	link	true	/test pull-etcd-coverage-report

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[k8s-ci-robot]

PR needs rebase.

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[ahrtr]

This PR doesn't have much benefit.

• Replaying 5K wal entries is far less than 1s
• Even we only replay WAL entries starting from consistent-index, we still need to load entries starting from the latest snapshot; otherwise it may cause a snapshot being sent to a follower which might be just lag behind just 1 or 2 entries.