WIP: Refactor Torrect checker

[xoriole]

Torrent Checker is broken down into two services:

1. CheckerService: This service is responsible for doing torrent checks using different trackers and DHT and does not deal with any database stuff.
2. DbService: This service is responsible for saving to and retrieving from the database the torrent health info. This service does not deal with any torrent checking itself.

With checker and persistence related functions delegated to these two services, TorrentChecker becomes a bit more simplified with the following interface

class TorrentChecker:

   # Main function that checks, persists and returns the health info for given infohash
    async def check_torrent_health(self, infohash: bytes, timeout=20, scrape_now=False) -> HealthInfo:

    # Periodic jobs defined in initialize()
    async def check_random_tracker(self):
    async def check_local_torrents(self) -> Tuple[List, List]:
    async def check_torrents_in_user_channel(self) -> List[Union[HealthInfo, BaseException]]:

    async def initialize(self):
        self.register_task("check random tracker", self.check_random_tracker, interval=TRACKER_SELECTION_INTERVAL)
        self.register_task("check local torrents", self.check_local_torrents, interval=TORRENT_SELECTION_INTERVAL)
        self.register_task("check channel torrents", self.check_torrents_in_user_channel, interval=USER_CHANNEL_TORRENT_SELECTION_INTERVAL)
        ...
        
    async def shutdown(self):