Enhancement: `getUserDB` method

[SukantGujar]

Please add a getUserDB(user_id, dbName, type) method which returns a PouchDB instance with admin access. This will allow the server API to easily manipulate the user DB with admin privileges. Without this API, the only way is to share the CouchDB admin credentials separately with the rest of the server code.

This will also go in parallel with other PouchDB instances provided by superlogin, like usersDB and couchAuthDB.