No standard way to do something in the background

[Stuart Langridge]

There are a few potential ways to run something time-consuming in the background in an Enso command. We should choose one and make it part of ensoapi, to make it easy for users to do this.

Note: this is *not* for population of valid_args. This is just if I want to, say, kick off a web request from a command which might take a few seconds to come back: Enso should not hang while the request is being made.