Add some config to sync_gateway to make sync_gateway more flexible and high scalability

[atom992]

I am build a app, the business logic is following:

one user belong to 0-n company, one company have some documents. 

I want to deploy couchbase and sync_gateway by company. 

so I can manage documents by company. such as cluster deploy, backup and restore by company.

I think it will be more flexible and high scalability.

As I know, sync_gateway will route document and control write/read document by channel, which depend on user and session(access()/requrieUser()). that means user document are coupling with 'company' documents

so, If sync_gateway can store user document and session document(may be) alone with other documents as public identity store,It will be possible to deploy sync_gateway and couchbase by company.

the config of sync_gateway database looks like:

```

"databases": {

  "db": {

    "server": "http://localhost:8091",

    "bucket": "default",

    "identity_server": "http://localhost:8091",

    "identity_bucket": "identity",    

    "users": { "GUEST": { "disabled": false, "admin_channels": ["*"] } }

  }

}

```

or may be there is a better way to make sync_gateway more flexible and high scalability?