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

generic-handlers.go

@@ -275,7 +275,6 @@ var notimplementedBucketResourceNames = map[string]bool{
 	"cors":           true,
 	"lifecycle":      true,
 	"logging":        true,
-	"notification":   true,
 	"replication":    true,
 	"tagging":        true,
 	"versions":       true,