Configurable mailboxes 404

PR for #404 
`WithUnboundedStash` and `WithBoundedStash` now inherits from `RequireMessageQueue`
Some initial work on creating configured mailboxes through the `Mailboxes` class.
e.g. Required message queue semantics can be resolved now.

This also makes the behavior of stashing work as intended, messages are prepended on the mailbox message queue.