Dynlink with Findlib support for long-running plugins

In the config file

```toml
[plugins]
   ocamlfind = ["some-package", "some-other-package"]
   compiled = "some_dyn_compiled_module.cmxs"
```

Ketrew will load at runtime the findlib packages and the compiled module.

There is one example of plugin: `src/test/dummy_plugin.ml`  and a workflow that uses it `dummy_plugin_user.ml`.

Documentation will come in a later PR (other things to document).

Issue #1 is known already; saved for later.





There is also more in `moadoc`:
https://github.com/hammerlab/ketrew/blob/moadoc/src/doc/Long-Running_Plugins.md

The inclusion itself, just takes all the functions of `Ketrew_daemonize` as "default" ones for this module (can we call that "module-level inheritance"?).