chore: unified the variable name for sync.WaitGroup to wg

[jiekun]

Describe Your Changes

follow-up of #709 per comment #709 (comment)

1. Renamed var wgWorkers sync.WaitGroup to wg.
2. Place wg.Done() at the beginning of the goroutine function with defer, instead of at the end of the function.
3. (minor) renamed variable i in for-loop to workerID. Because it conflict with for i := range bsws in the goroutine. While it does not affect the execution, there's nothing bad to make it clear and unique.

Again, the CHANGELOG is left empty intentionally.

Checklist

The following checks are mandatory:

• My change adheres to VictoriaMetrics contributing guidelines.
• My change adheres to VictoriaMetrics development goals.

[Copilot]

Pull Request Overview

This PR standardizes the naming convention for sync.WaitGroup variables to wg and improves goroutine cleanup patterns. The changes follow up on PR #709 to maintain consistency across the codebase.

• Renamed wgWorkers to wg for consistency with project naming conventions
• Moved wg.Done() to use defer pattern at the beginning of goroutines for better cleanup
• Renamed loop variable i to workerID to avoid naming conflicts and improve clarity

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[valyala]

@jiekun , thank you for the pull request, which improves the consistency of VictoriaLogs source code!