Overhaul core Tracker: extract torrents context (part 1)

[josecelano]

Parent issue: #1181

Remove field torrents from core::Tracker extracting new services for each responsibility:

• InMemoryTorrentRepository: all the info about torrents in memory.
• DatabasePersistentTorrentRepository: the torrent's data that is persisted (only number of downloads)
• TorrentsManager: other tasks like load torrents' data from database, ...
• ... there could be other services after the refactor.