Split flow executor out as service from flow-service

[zootalures]

Flow service should only operate on data by references, this means there has to be something to pass in data to (external) function calls and store results,

This should collaborate with the blob store see #84

[msgodf]

@owencliffe is this something you're doing as part of the work on #84? Either way, I think it's dependent on it?

[zootalures]

Not specifically but #84 is sufficiently hairy you can't do this till after it's done.