fix: Properly reset wal segment writer

[cyriltovena]

What this PR does / why we need it:

We recently discover that we were holding on entries and stream labels. This fixes the reset of wal segment writer.

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• Title matches the required conventional commits format, see here
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