Deamon worker

[gpetretto]

In connection with the idea of quickly executing short/small jobs (materialsproject/jobflow#546), I was thinking that in some cases the overhead of all the procedure of uploading, submitting, checking of the job can be avoided. Right now the worker types are local and remote and I was considering adding a worker of type daemon. Meaning that the Job will be executed directly in the process of the daemon. I am not sure if this should be a type that a user can configure, or an internal worker that is always present.
The pros that I see are:

• quick execution for small jobs
• avoiding useless connections to remote host
• avoiding dumping input output files locally or remotely

Cons I could think of:

• it could block (part of) the Runner if a Job gets stuck or if lasts longer than expected.
• little control on the process: there will be no way of killing it if needed, unless killing the daemon altogether
• more momory and CPU consumed by the daemon.

I would like to gather a few comments before deciding if it is worth implementing this feature.

[Andrew-S-Rosen]

In jobflow + FireWorks, the Response(replace) action was its own job, which I always found a bit cumbersome (but understandable from a workflow perspective). If that's the same in jobflow-remote, this seems like an excellent candidate to have automatically run via the daemon.