[Core] implement synchronous and v2 metafile creation

[rcarpa]

Assuming an entity which has access to deluge daemons on multiple storage servers,
this will allow to generate a torrent file on a source server, followed by triggering
the transfer from multiple source locations towards the destination storage server.

In particular, I'm interested to add support for deluge/bittorent as a wire transfer protocol in rucio

The individual commit messages contain additional information about the changes.

[cas--]

Cannot import libtorrent here, this burdens UI clients with libtorrent requirement

[rcarpa]

I imported libtorrent in the corresponding function only

[cas--]

I don't feel it is a good idea to have these libtorrent implementation details leaking into the functions and API. It would be better to use something like a TorrentFormat Enum and have a format param that can accept str or Enum.

A tentative implementation attempt:

class TorrentFormat(str, Enum):
     V1 = 'v1_only'
     V2 = 'v2_only'
     HYBRID = 'hybrid'

     @classmethod
     def _missing_(cls, value):
         if not value:
             return cls.V1

         value = value.lower()
         for member in cls:
             if member.value.startswith(value):
                return member

     def to_lt_flag(self):
         if self.value == 'v1_only':
             return 64
         if self.value == 'v2_only':
             return 32
         return 0

This can be better represent available options to users.

[rcarpa]

Did it a bit differently. The called it torrent_format everywhere to avoid clash with the builtin format function. The format names are v1, v2 and hybrid (without the _only suffix). And enforce a valid format string in missing. Instead of returning v1 on invalid format.

[rcarpa]

Sorry for taking so long. Was overwhelmed by a couple of more urgent tasks.

[rcarpa]

@cas-- , would it be possible to create a release ? (could be an -dev or an -alpha release). It would be very useful to be able to use this new features by just installing deluge from pip.