Fix nullptr when mix mode change to only v3.

[jiaqizho]

What problem does this PR solve?

Issue Number: close #4893

Problem Summary:

• When we changed MIX_MODE to ONLY_V3, we will change the run_mode at last.
• Then we will used PageReaderImplMixed/PageWriterImplMixed to read/write page. But in that ps_v2 will be nullptr.

What is changed and how it works?

• make writer and reader not used Mixed, if we do change MIX_MODE to ONLY_V3.

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

None

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• JaySon-Huang
• hehechen

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[JaySon-Huang]

LGTM

[JaySon-Huang]

/merge

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This pull request has been accepted and is ready to merge.

Commit hash: f5edcaa

[sre-bot]

Coverage for changed files

Filename                            Regions    Missed Regions     Cover   Functions  Missed Functions  Executed       Lines      Missed Lines     Cover    Branches   Missed Branches     Cover
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
DeltaMerge/StoragePool.cpp              173                67    61.27%          29                 6    79.31%         439               171    61.05%         114                56    50.88%
Transaction/RegionPersister.cpp         159                80    49.69%          15                 2    86.67%         297               130    56.23%          84                48    42.86%
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
TOTAL                                   332               147    55.72%          44                 8    81.82%         736               301    59.10%         198               104    47.47%

Coverage summary

Functions  MissedFunctions  Executed  Lines   MissedLines  Cover
18150      9790             46.06%    202789  98044        51.65%

full coverage report (for internal network access only)