Error when set security of database: no_ring

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

## Description

When I use `curl` to set db security, the response is:
```
{"error":"error","reason":"no_ring"}
```
And the couchdb logs show that:
```
rpc couch_db:set_security/2 badarg
Failed to set security object on ... :: {error,badarg}
```
But other databases are ok.

The data size of the error db is about 1GB.

## Steps to Reproduce

[NOTE]: # ( Include commands to reproduce, if possible. curl is preferred. )
The command I used is just like
```
curl http://localhost:5984/db_name/_security -X PUT -H 'content-type: application/json' -H 'accept: application/json' -d '{"admins":{"names":["admin"],"roles":[]},"members":{"names": ["admin"],"roles": []}}'
```

## 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: docker image with version 2.1.1
```
{"couchdb":"Welcome","version":"2.1.1","features":["scheduler"],"vendor":{"name":"The Apache Software Foundation"}}
```

Start by reproducing the PUT request to /db_name/_security against the reported large database and inspect the couch_db:set_security/2 failure in the logs. Trace the no_ring/badarg response from that endpoint and verify that setting the security object succeeds for the affected database without regressing other databases.

Written by the indexing model from the issue text.