Query buffering, terminating blocking transactions for INSTANT DDL and other "special plans"

[shlomi-noach]

Description

Fixes #17944

This PR introduces query buffering and forced query/transaction termination for special plan operations, which at this time include INSTANT DDL and range partition rotation.

• When a migration is deemed eligible to run as a "special operation" (is instant-able and --prefer-instant-ddl strategy flag is supplied, or is found to be a range partition rotation change)
• We now apply query buffering around the actual change
• In addition, if --force-cut-over-duration flag is nonzero, and irrespective of its value, we also issue a one time termination for queries and transactions holding locks or metadata locks on the migrated table, just before issuing the ALTER TABLE statement.
  In the future we may also consider terminating queries/transactions while a migration is being stalled.

Related Issue(s)

#17944

Documentation: vitessio/website#1990

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
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• Tests were added or are not required
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• Documentation was added or is not required

Deployment Notes

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[codecov]

Codecov Report

Attention: Patch coverage is 0% with 54 lines in your changes missing coverage. Please review.

Project coverage is 67.54%. Comparing base (0bf4276) to head (f178c99).
Report is 100 commits behind head on main.

Files with missing lines	Patch %	Lines
go/vt/vttablet/onlineddl/executor.go	0.00%	54 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #17945      +/-   ##
==========================================
- Coverage   67.56%   67.54%   -0.03%     
==========================================
  Files        1597     1597              
  Lines      259763   259849      +86     
==========================================
+ Hits       175506   175507       +1     
- Misses      84257    84342      +85     

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[shlomi-noach]

Documentation PR: vitessio/website#1990