[release-3.5] server: set multiple concurrentReadTx instances share one txReadBuffer.

[gyuho]

Backport #12933

@wilsonwang371

[codecov-commenter]

Codecov Report

Merging #13036 (7a7d6f9) into release-3.5 (c0d1450) will increase coverage by 0.21%.
The diff coverage is 100.00%.

@@               Coverage Diff               @@
##           release-3.5   #13036      +/-   ##
===============================================
+ Coverage        62.70%   62.92%   +0.21%     
===============================================
  Files              432      421      -11     
  Lines            33354    33598     +244     
===============================================
+ Hits             20915    21141     +226     
+ Misses           10359    10354       -5     
- Partials          2080     2103      +23     

Flag	Coverage Δ
all	62.92% <100.00%> (+0.21%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
server/mvcc/backend/backend.go	79.85% <100.00%> (+2.21%)	⬆️
server/mvcc/backend/tx_buffer.go	89.74% <100.00%> (+0.26%)	⬆️
raft/quorum/quorum.go	0.00% <0.00%> (-100.00%)	⬇️
raft/tracker/state.go	0.00% <0.00%> (-100.00%)	⬇️
raft/util.go	20.56% <0.00%> (-65.43%)	⬇️
raft/confchange/restore.go	39.53% <0.00%> (-55.82%)	⬇️
raft/raftpb/confchange.go	22.97% <0.00%> (-52.71%)	⬇️
raft/quorum/voteresult_string.go	0.00% <0.00%> (-50.00%)	⬇️
raft/quorum/majority.go	60.75% <0.00%> (-39.25%)	⬇️
etcdctl/ctlv3/command/watch_command.go	5.29% <0.00%> (-37.65%)	⬇️
... and 91 more

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