Split other methods in txn into separate files

[serathius]

/cc @ahrtr @fuweid @siyuanfoundation

[fuweid]

Will revisit this pr after #19811 merged. Overall, it looks good.

[serathius]

Done

[fuweid]

LGTM on green

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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

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[codecov]

Codecov Report

Attention: Patch coverage is 91.70984% with 16 lines in your changes missing coverage. Please review.

Project coverage is 68.87%. Comparing base (1a3a5a4) to head (ef21176).
Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
server/etcdserver/txn/range.go	90.82%	10 Missing ⚠️
server/etcdserver/txn/put.go	92.98%	2 Missing and 2 partials ⚠️
server/etcdserver/txn/delete.go	92.59%	1 Missing and 1 partial ⚠️

Additional details and impacted files

Files with missing lines	Coverage Δ
server/etcdserver/txn/txn.go	94.67% <ø> (+0.81%)	⬆️
server/etcdserver/txn/delete.go	92.59% <92.59%> (ø)
server/etcdserver/txn/put.go	92.98% <92.98%> (ø)
server/etcdserver/txn/range.go	90.82% <90.82%> (ø)

... and 35 files with indirect coverage changes

@@            Coverage Diff             @@
##             main   #19826      +/-   ##
==========================================
+ Coverage   68.54%   68.87%   +0.32%     
==========================================
  Files         416      424       +8     
  Lines       35677    35876     +199     
==========================================
+ Hits        24456    24710     +254     
+ Misses       9786     9739      -47     
+ Partials     1435     1427       -8     

---

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