Cleanup `Request.is_ready` logic

[toots]

Request.is_ready semantics is unclear and buggy. See: https://github.com/savonet/liquidsoap/runs/5801028774?check_suite_focus=true

This is due to a couple of facts:

• We are using the request's data to determine if it is ready. This should be a state
• We have overloaded requests from originally representing a decodable media file to representing any downloaded file
• Hence, the logic for is_ready says:

let is_ready t =
  t.indicators <> []
  && Sys.file_exists (peek_indicator t).string
  && (t.decoder <> None || t.ctype = None)

The (t.decoder <> None || t.ctype = None) condition is here to validate requests that are not media files (t.ctype = None). However, if a request represents a local file (Sys.file_exists (peek_indicator t).string = true) then it will always be considered ready.

Proposed solution:

• Use a clear state for request resolution and implement it as a state machine.
• Get rid of requests as an API to download files. We now support proper file downloading via http.get streaming requests and file.write callbacks.