Be able to clean-up `Logs.Src.list`

[dinosaure]

Currently, when a source is created, it is stored in a global list. If a program attempts to create multiple sources "on the fly" based on dynamic parameters such as:

let gen () =
  let v = Atomic.make 0 in
  fun () -> Atomic.fetch_and_add v

(* spawn every time a new client arrive. *)
let handler_client () =
  let src = Logs.Src.create (Fmt.str "client-%08x" (gen ()) in
  do_stuff ~src ()

We end up with a bunch of sources that may be unused but are still kept in memory. It may be useful to either have a way to delete a source as soon as it is no longer needed (something like lwt-dllist can be useful), or to specify in the documentation that a source will remain active "forever" even if it is no longer used.

[dinosaure]

@hannesm points me about tags, I will look into it but it's probably good to add into the documentation that a created source is kept forever.

[dbuenzli]

I think the idea was that you'd mainly have a bunch of sources defined at the beginning of your program until the end. The global list is here so that the application can always be in control of log message emitters even if it may not know where they are defined.

This could be relaxed but would likely need additional concepts to the library (e.g. an active vs inactive source) and I'm not sure it's worth adding more conceptual complexity here.