This important commit introduces the 'DbCmd' constraint set, allowing DB
functions to require only a connection pool and some `GargConfig`.

Merge remote-tracking branch 'origin/239-readme-test-reinstall-project-with-cabal-and-update-the-readme-file-2' into dev

This is to prevent accidental updates of hyperdata with other JSON
structs.

The bug was introduced in

!177 (diffs)

It caused hyperdata to be updated with whole corpus, serialized, hence
hyperdata inside hyperdata.

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.