accelerated alba: cache on read slows down download of fragments (way too much)

[toolslive]

(*alba_client_download.ml: line 200 *)
     begin
       if cache_on_read && fragment_id < k (* only cache data fragments *)
       then
         fragment_cache # add
                        namespace_id
                        cache_key
                        (Bigstring_slice.wrap_bigstring maybe_decompressed)
       else
         Lwt.return []
     end >>= fun mfs -> ...

we're waiting until the fragment was added to the fragment cache
(which might take long in case it's alba based)

[toolslive]

alba 1.3.8