api: Implement bucket notification. (#2271)

* Implement basic S3 notifications through queues

Supports multiple queues and three basic queue types:

1. NilQueue -- messages don't get sent anywhere
2. LogQueue -- messages get logged
3. AmqpQueue -- messages are sent to an AMQP queue

* api: Implement bucket notification.

Supports two different queue types

- AMQP
- ElasticSearch.

* Add support for redis

vendor/gopkg.in/olivere/elastic.v3/search_queries_exists.go

@@ -0,0 +1,49 @@
+// Copyright 2012-2015 Oliver Eilhard. All rights reserved.
+// Use of this source code is governed by a MIT-license.
+// See http://olivere.mit-license.org/license.txt for details.
+
+package elastic
+
+// ExistsQuery is a query that only matches on documents that the field
+// has a value in them.
+//
+// For more details, see:
+// https://www.elastic.co/guide/en/elasticsearch/reference/current/query-dsl-exists-query.html
+type ExistsQuery struct {
+	name      string
+	queryName string
+}
+
+// NewExistsQuery creates and initializes a new dis max query.
+func NewExistsQuery(name string) *ExistsQuery {
+	return &ExistsQuery{
+		name: name,
+	}
+}
+
+// QueryName sets the query name for the filter that can be used
+// when searching for matched queries per hit.
+func (q *ExistsQuery) QueryName(queryName string) *ExistsQuery {
+	q.queryName = queryName
+	return q
+}
+
+// Source returns the JSON serializable content for this query.
+func (q *ExistsQuery) Source() (interface{}, error) {
+	// {
+	//   "exists" : {
+	//     "field" : "user"
+	//   }
+	// }
+
+	query := make(map[string]interface{})
+	params := make(map[string]interface{})
+	query["exists"] = params
+
+	params["field"] = q.name
+	if q.queryName != "" {
+		params["_name"] = q.queryName
+	}
+
+	return query, nil
+}