Indexer - Create API to store the bucket password

With the UPR integration the view-engine receives the items through an UPR stream, hence it needs to authenticate a bucket and also know it's password in case it's password protected. ns_server knows that password and needs to notify the view engine when it gets set/updated.

The task is to provide an API in couchdb that ns_server can use to notify couchdb about a password change. it will be per bucket and couchdb needs to notify all view groups about that change.