gh-159690: Fix TimedRotatingFileHandler stopping rollovers after a failed or skipped rollover

[lazerg]

doRollover() only updated rolloverAt on its last line, so an early return (backup name already exists, as happens at DST changes) or an exception after the rename left it in the past. Every later emit() then hit the same early return and the handler never rolled over again. Now rolloverAt is computed as soon as the backup name is known, so only that one rollover is affected.

• Issue: TimedRotatingFileHandler stops rolling over permanently once doRollover() exits without updating rolloverAt #159690

[picnixz]

I'm going to close this because the OP may have wanted to open a PR themselves. Please don't let any running agent snipe issues to create PRs automatically. Or at least, ask on the issue if the OP wants to make a PR.