txthrottler: further code cleanup

[timvaillancourt]

Description

Building on top of #12901, this PR cleans up go/vt/vttablet/tabletserver/txthrottler by:

1. Reordering structs based on @deepthi's recommendation in Cleanup panics in txthrottler, reorder for readability #12901:

   I would move the structs up to the beginning AND reorder the struct methods so that state and config methods are defined right after the struct definition of TxThrottler. Such wholesale reorganization will definitely make the PR harder to review, so you might want to get the panic changes all reviewed and squared away and then do the re-org as a separate commit.

2. Removing *topo.Server from txThrottlerConfig struct because *topo.Server is not really "configuration"
   • Added *topo.Server to TxThrottler struct and passed down instead
3. Moving func init() to be after the func it calls: func resetTxThrottlerFactories()

Related Issue(s)

#12901

Checklist

• "Backport to:" labels have been added if this change should be back-ported
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on the CI
• Documentation was added or is not required

Deployment Notes

[vitess-bot]

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[harshit-gangal]

@timvaillancourt There is a conflict that needs to be resolved.

[timvaillancourt]

@harshit-gangal conflict resolved! 🙇