extension: Add MessageStored event (#316)

* Replace existing direct StoreManager->msghub communication with this
  event
* For #280 #309 #312 #310

Signed-off-by: James Hillyerd <james@hillyerd.com>

Signed-off-by: James Hillyerd <james@hillyerd.com>

pkg/extension/event/events.go

@@ -0,0 +1,17 @@
+package event
+
+import (
+	"net/mail"
+	"time"
+)
+
+// MessageMetadata contains the basic header data for a message event.
+type MessageMetadata struct {
+	Mailbox string
+	ID      string
+	From    *mail.Address
+	To      []*mail.Address
+	Date    time.Time
+	Subject string
+	Size    int64
+}