Require engines to pass clones of messages to handlers.

[jmalloc]

In a nutshell, to prevent problems with memory being shared across messages and within aggregate/process state, we could require any message value passed to a handler to be a clone of the message such that we can say that the handler "owns" the memory used by values within the message.

This would apply regardless of which method on the handler is called, e.g. RouteCommandToInstance, HandleCommand, TimeoutHint, etc.