Refactor service: torrust_index::services::torrent::Index

[josecelano]

Relates to: #494 (comment)

The torrent Index service is too big and it has too many dependencies. That makes it harder to test it.

I would suggest using a command/handler pattern. One command per action.

Torrent:

• Add torrent.
• Delete torrent.
• Get torrent (the metainfo file).

Torrent index metadata:

• Get torrent info (the metadata in the index).
• Update torrent info.
• Search torrents (generate torrent info listing).

Torrent statistics:

• Import torrent statistics from the tracker.

@torrust/torrustaceans