Use bounded mail boxes for xtra::Actors

[luckysori]

This would be an attempt at solving some of the issues described here: #1954.

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Before tackling this, consider remodelling the way we handle internal actor tasks. At the moment, actors send messages to themselves at an interval. This could have a negative interaction with the introduction of bounded inboxes. @klochowicz suggested substituting this mechanism with spawned tasks with an internal loop.

[luckysori]

We are not sure what to do with this yet.