Event persistence for MQTT (#7268)

- The events will be persisted in queueStore if `queueDir` is set.
- Else, if queueDir is not set events persist in memory.

The events are replayed back when the mqtt broker is back online.

pkg/event/target/store.go

@@ -0,0 +1,37 @@
+/*
+ * Minio Cloud Storage, (C) 2019 Minio, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package target
+
+import (
+	"errors"
+	"github.com/minio/minio/pkg/event"
+)
+
+// ErrLimitExceeded error is sent when the maximum limit is reached.
+var ErrLimitExceeded = errors.New("[Store] The maximum limit reached")
+
+// ErrNoSuchKey error is sent in Get when the key is not found.
+var ErrNoSuchKey = errors.New("[Store] No such key found")
+
+// Store - To persist the events.
+type Store interface {
+	Put(event event.Event) error
+	Get(key string) (event.Event, error)
+	ListAll() []string
+	Del(key string)
+	Open() error
+}