Optimize fetching threads

[slatinsky]

By adding new threads support, Fetching channels... stage takes too long for bigger guilds with many threads. Some data doesn't need to be fetched, because we know, that those channels/threads won't be exported.

If --after parameter is specified, the main speed-up is to break early of thread fetching, by sorting threads by LastMessageId (endpoint channels/{channel.Id}/threads/search - query parameters sort_by=last_message_time&sort_order=desc) and comparing After and LastMessageId. This check is implemented only for user tokens

Skip thread export for channel if:

• channel.Kind == ChannelKind.GuildCategory - categories never have threads
• channel.Kind.IsVoice() - voice channels never have threads
• channel.LastMessageId - if the channel is empty, the channel doesn't have threads (if the channel is forum post, LastMessageId will be channel.Id of the last forum post)
• (--before) Before < channel.Id - if the channel is created in the future

Skip export of these ordinary channels if:

• After > channel.LastMessageId - the channel doesn't have new messages (BUT it can have new messages in threads, that's why we don't skip it in thread exports)

---

Potential problems:

• channels skipped this way won't be printed in section Failed to export XXX channel(s):
• Old bug - timestamp supplied using --before can overflow into the future - for example --before "2010-08-30T00:00:00.000Z" overflows to snowflake 16548918821562351616 (2140-01-12T07:35:11.104Z). By adding more early checks, there may be more cases, where this bug can be an issue

Tested - CLI json export using exportGuild with --include-threads All and combinations of --before and --after parameters

[Tyrrrz]

Thanks!

[Tyrrrz]

• Old bug - timestamp supplied using --before can overflow into the future - for example --before "2010-08-30T00:00:00.000Z" overflows to snowflake 16548918821562351616 (2140-01-12T07:35:11.104Z). By adding more early checks, there may be more cases, where this bug can be an issue

Can you please also make a PR for that in Snowflake.FromDate(...) method (or wherever applicable)? We can assume that a valid snowflake should be at least after Discord was launched.