Online DDL: better support for subsecond --force-cut-over-after DDL strategy flag value.

[shlomi-noach]

Description

The --force-cut-over-after DDL strategy flag says "if we do a cut-over, how much time since the migration was ready to complete needs to pass before we force the cut-over".

Normally we'd put values like --force-cut-over-after=1h to give the migration some chance to complete normally, but then at 1h be aggressive about cutting over.

In testing, we sometimes want to force the cut-over on the very first attempt. Production users may want that, too. So we want to e.g. provide --force-cut-over-after=1ns (one nanosecond). However, we currently calculate the time since ready_to_complete_timestamp in the granularity of seconds. For testing in particular, but also possible for production, it is possible that the very first cut-over attempt happens within the same second range of ready_to_complete_timestamp , which evaluates the time passed since ready_to_complete_timestamp as zero.

The value of zero is of course always less than any nonzero value for --force-cut-over-after, so even a nanosecond fails the comparison, and this means the first cut-over attempt is not forced (ie does not kill queries & transactions).

This PR fixes the issue by reading the timestamp difference in microseconds. The column value of ready_to_complete_timestamp is still seconds, but the "time since" evaluation compares the value with now(6), which practically always produces a nonzero value.

With this change a value of --force-cut-over-after=1ns can be safely trusted to ensure even the very first cut-over attempt is forced.

Related Issue(s)

Fixes #16634

Checklist

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[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 68.98%. Comparing base (538dd4c) to head (5e36965).
Report is 3 commits behind head on main.

Files	Patch %	Lines
go/vt/vttablet/onlineddl/executor.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #16635   +/-   ##
=======================================
  Coverage   68.98%   68.98%           
=======================================
  Files        1562     1562           
  Lines      200690   200690           
=======================================
+ Hits       138449   138450    +1     
+ Misses      62241    62240    -1     

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