close() not unlock all files after invoke createIndex

### Issue
First, run the codes below
```
let testdb
!(async () => {
testdb = new PouchDB('test', {
adapter: 'leveldb',
prefix: './.test/',
})
try {
await testdb.createIndex({ index: { fields: ['doc_type'] } })
} catch (err) {
console.log(err)
}
await testdb.close()
})()
```

Then, delete **.test** folder. If you are using windows, you will see a error window with sth like '000003.log is open in Node.js Javascript Runtime'.

### Info
- Environment: Node.js
- Platform: Window10
- Adapter: leveldb
- Server: PouchDB Local
- PouchDB version: 9.0.0

### More info
I tried to remove **createIndex**, and then delete the folder, no error came out. So, i guess the problem is **createIndex**.
```
let testdb
!(async () => {
testdb = new PouchDB('test', {
adapter: 'leveldb',
prefix: './.test/',
})
await testdb.close()
})()
```