Implement the new event logging framework

This "includes" the work on !706 (closed) and develops it further to make it able to be the main logging mechanism while the “legacy-logging” (lwt-log-based) is still supported but reimplemented within tezos-event-logging.

A few proper events are already in there (e.g. Client_baking_blocks, Lwt_utils.worker, …), the next steps will be to replace all the Legacy_logging.Make_semantic with structured events and removing the almost empty *_logging modules.

This MR provides 2 event “sinks:” the legacy lwt-log channels + the File_event_sink module (and the commands in tezos-admin-client). Later, we'll add at least: a “forward on unix-file-desc” sink and a “keep everything in memory for X minutes in case of error” sink.

Notes:

• Fixing the concurrency bug in Lwt_utils_unix.create_dir could be a separate MR if you think it is urgent.
• I kept backwards compatibility in src/bin_node/node_config_file.ml so the lwt-log-sink configuration is still called log, let me know if we should break this.
• The Scriptable module comes from !773 (closed).