Intermittent "file already closed" error when creating new segment file for wal

Reported in community slack [here](https://influxcommunity.slack.com/archives/CH7C56X4Z/p1615973626007200).

This error occurs "regularly" after running InfluxDB v2 (2.0.4) for around a month:
```
influxdbv2/engine/wal//autogen//_.wal: file already closed
```

The logs immediately preceding the error are:
```
Cache snapshot (start)
Cache snapshot (end)
Error writing snapshot
```

Start with the WAL path shown in the error, influxdbv2/engine/wal//autogen//_.wal, and trace new segment creation alongside the preceding cache snapshot messages. Reproduce the month-scale failure if possible and determine why snapshot writing encounters a closed file; done means the reported error no longer occurs during snapshot and segment creation.

Written by the indexing model from the issue text.