couch_peruser creates databases only sporadically

[NOTE]: # ( ^^ Provide a general summary of the issue in the title above. ^^ )

## Description
With `couch_peruser=true ` databases are created only sporadically. I have created 10 users and only 1 of them has received its own database. I don't know if there is a naming convention that must be followed?

[NOTE]: # ( Describe the problem you're encountering. )
[TIP]: # ( Do NOT give us access or passwords to your actual CouchDB! )

## Steps to Reproduce

- setup local couchDB on Windows
- set `couch_peruser=true `
- generate some users with admin account

I used:
`PUT http://127.0.0.1:5984/_users/org.couchdb.user:78946`

with:

```
{
"name": "78946",
"type": "user",
"roles": [],
"password": "pass"
}
```

I used 10 different numbers as username.
The users were created perfectly and I have 10 users in the _users database.

[NOTE]: # ( Include commands to reproduce, if possible. curl is preferred. )

## Expected Behaviour

I expected 10 private databases - one per user... But I only got one database.

[NOTE]: # ( Tell us what you expected to happen. )

## Your Environment

[TIP]: # ( Include as many relevant details about your environment as possible. )
[TIP]: # ( You can paste the output of curl http://YOUR-COUCHDB:5984/ here. )

* CouchDB version used: 3.1.1 and 3.0.0 (tried both)
* Browser name and version: Chrome 86
* Operating system and version: Windows 10

## Additional Context

[TIP]: # ( Add any other context about the problem here. )

Start by reproducing the report on CouchDB 3.0.0 or 3.1.1 with couch_peruser=true, creating several users through the shown PUT request. Compare the _users records with the private databases that are created and inspect the per-user database creation path. Done means every created user consistently receives one private database, or the required naming convention is documented.

Written by the indexing model from the issue text.