[Core] Support creating v2 torrents

Add support for v2 torrents in create_torrent, but keep the old
default of only adding the v1 metadata.

Unify the single-file and directory cases to avoid code
duplication.

V2 torrents require files to be piece-aligned. The same for
hybrid v1/v2 ones. To handle both cases of piece-aligned and
non-aligned files, always read the files in piece-aligned
chunks. Re-slice the buffer if needed (for v1-only multi-file
torrents).

Also, had to adapt to progress event. It now depends on the
number of bytes hashed rather than the number of pieces. To
avoid sending and excessive amount of event when handling a
directory with many small files, add a mechanism to limit
event period at 1 per piece_length.

Closes: #430

deluge/core/core.py

@@ -21,7 +21,7 @@
 
 import deluge.common
 import deluge.component as component
-from deluge import path_chooser_common
+from deluge import metafile, path_chooser_common
 from deluge._libtorrent import LT_VERSION, lt
 from deluge.configmanager import ConfigManager, get_config_dir
 from deluge.core.alertmanager import AlertManager
@@ -998,7 +998,11 @@ def create_torrent(
         created_by=None,
         trackers=None,
         add_to_session=False,
+        torrent_format=metafile.TorrentFormat.V1,
     ):
+        if isinstance(torrent_format, str):
+            torrent_format = metafile.TorrentFormat(torrent_format)
+
         log.debug('creating torrent..')
         return threads.deferToThread(
             self._create_torrent_thread,
@@ -1012,6 +1016,7 @@ def create_torrent(
             created_by=created_by,
             trackers=trackers,
             add_to_session=add_to_session,
+            torrent_format=torrent_format,
         )
 
     def _create_torrent_thread(
@@ -1026,6 +1031,7 @@ def _create_torrent_thread(
         created_by,
         trackers,
         add_to_session,
+        torrent_format,
     ):
         from deluge import metafile
 
@@ -1038,6 +1044,7 @@ def _create_torrent_thread(
             private=private,
             created_by=created_by,
             trackers=trackers,
+            torrent_format=torrent_format,
         )
 
         write_file = False