Fixes #258

This commit fixes the issue of the "empty Ngrams". The culprit was a bug
in the `arxiv-api` package, which was fixed by:

gargantext/crawlers/arxiv-api!2 (commits)

In a nutshell, due to the fact the relevant `Conduit` was never
completing, it was blocking the `flow` function, which was endlessly
waiting for the last part of the results to arrive.

Hence we were never calling the `flowCorpusUser`, which is the function
responsible for generating the Ngrams.

This commit moves the `threadDelay` present in the `gcThread` function
at the beginning of the `go` aux rather than at the end. This has the
effect that the first GC happens _after_ the delay time and not
immediately, which might make some tests unpredictable if we get unlucky
and some job ids are removed from the underlying map before we can
inspect their state.

We need to fix them properly as part of #198.

This reverts merge request !166

JSON should now be better so that the UI can handle it in task async
handler.

JSON should now be better so that the UI can handle it in task async
handler.

This big commit improves the jobs API in a way that now we can
completely abstract away over a JobLog.

This commit also moves the JobHandle into the `Gargantext.Utils.Jobs.Monad` module.

This commit renames the `Gargantext.Utils.Job.API` module to
`Gargantext.Utils.Job.Internal`, because it turns out it's *not* what
Gargantext is using directly; rather, the user-facing function in use
lives at `Gargantext.Utils.Jobs` and is called `serveJobsAPI`. This
distinction makes clearer who is using what.

Based on the suggestions in
https://kodimensional.dev/space-leak