<code>receive</code> method of an actor.
    </p><p>
    Its main purpose is to maintain a <code>Map</code> of
    <a href="#code/src/main/scala/sample/eventuate/OrderActor.scala" class="shortcut">OrderActors</a>
    by id each one representing a specific
    order and dispatch <code>OrderCommand</code>s or a <code>Resolve</code>-command to them
    (to modify the order) or a <code>SaveSnapshot</code>-command for taking a
    <a href="http://rbmhtechnology.github.io/eventuate/reference/event-sourcing.html#s