Fetch and store an applications bytecode and store in SharedData

[snario]

To implement #478 we need to call provider.getCode for some appDefinition and then store that bytecode in the SharedData so that it can be referenced later when we want to do a valid transition check. This task tracks to fetching + storage of that bytecode.

Presumably this should be done when you create a channel for the first time or join a channel as a forked process.