flexible network sandboxes (a.k.a. sandbox tests in OCaml)

See the file ./src/sandboxes/README.md (→ https://gitlab.com/obsidian.systems/tezos/blob/sm@flexible-sandboxes/src/sandboxes/README.md)

EDIT: This is not true any more, I rebased on top of master:

This experiment is on top of !798 (merged) (it is not a strong dependency yet but it kind-of assumes that it is using the File_event_sink).
So it starts at 2833d6138e51b64641e2060ebca487f42821f216.