Add configurable mailbox message cap

- Add new configuration option [datastore]mailbox.message.cap
- Modify filestore to enforce message cap if value > 0
- Filestore unit tests for message cap when enabled & disabled
- Change to DataStore.Mailbox.NewMessage() interface to allow error
  return

smtpd/datastore.go

@@ -16,7 +16,7 @@ type Mailbox interface {
 	GetMessages() ([]Message, error)
 	GetMessage(id string) (Message, error)
 	Purge() error
-	NewMessage() Message
+	NewMessage() (Message, error)
 	String() string
 }