CommandDispatcher cannot optimize marshalling of user commands

Currently, all CommandDispatcher instances created from a given CommandDispatcherFactory share a single marshaller. This means that the marshaller used by WF for its internal commands is the same as the one used to marshal user commands. This limits the extent to which commands may optimize their marshalling, or indeed, use a different marshaller (e.g. ProtoStream).
We can remove the limitations above by using a distinct mashaller for each CommandDispatcher.