This MR introduces a new job queue system, implemented by the Gargantext.Utils.Jobs.* modules.

• Queue implements the actual queue(s). We have one queue per "job type" and will draw the next job to run from the highest priority job type's queue.
• Map implements the job ID => job data mapping.
• State gathers the two in a single data type
• Monad offers some interface and operations that can work in any monad that has a job environment of sorts
• API offers an HTTP interface to all of this that is API-compatible with servant-job's async stuffs.

This patch changes all uses of servant-job's serveJobsAPI to the new queue system's serveJobsAPI, which is almost identical: it just takes as additional argument the job type.

This system also comes with a modest but useful-starting-point test suite that looks at basic properties.

PS: I'm also removing gargantext-client, it's just too painfully slow to even load in ghci, let alone compile. I'll resurrect it if we need it some day, but let's not weight ourselves down until then.

@anoe Could you give a shot at using this branch to see how it goes? Feel free to go crazy and spawn a bunch of stuffs.