CBG-2977 allow DELETE on a broken DB config

[torcolvin]

CBG-2977

• This only works with a bootstrap connection, because otherwise you'd just modify the file on disk. This means that this only has test coverage where we use persistent config enabled. Also, persistent config isn't enabled for walrus.
• Added some boilerplate to allow LeakyBucket and TestBucket to support DropDataStore and CreateDataStore on the assumption this test would work with walrus. It does not though, because it relies on the bootstrap connection.
• Trying to delete a config without a bootstrap connection will result in a 404 since you wouldn't be able start up sync gateway with a bad config.
• the only non test changes are in rest/config.go and rest/handler.go.

Pre-review checklist

• Removed debug logging (fmt.Print, log.Print, ...)
• Logging sensitive data? Make sure it's tagged (e.g. base.UD(docID), base.MD(dbName))
• Updated relevant information in the API specifications (such as endpoint descriptions, schemas, ...) in docs/api

Integration Tests

• GSI=true,xattrs=true https://jenkins.sgwdev.com/job/SyncGateway-Integration/1814/

[adamcfraser]

I don't think the current approach works with admin auth enabled, added some suggestions about how we might address.

[adamcfraser]

There's some redundant work here - GetInactiveDatabase would have just done a fetchDatabase in the case where it didn't find the loaded db, and only returns 'not found' when the associated applyConfig fails, so we don't need another scan over all the buckets looking for the db config. Seems like we could modify GetInactiveDatabase to return another flag for 'dbConfigExists' that we could check here instead.

[adamcfraser]

One request on how we use options, otherwise looks good.