Design and implement a history crawler

[strinking]

Design and implement a history crawler

[emmiegit]

This issue also includes making the sql object thread-safe. So some sort of locking should be implemented to ensure that the sql object is not mutated by multiple users at once.

This will likely involve using asyncio.lock and/or asyncio.event

[emmiegit]

I have created the crawler branch. I will limit my implementation until the design is more solidified, but there is some code that can be written regardless.

My proposal for the crawler is that it runs as an asyncio task that plays round-robin for all the channels in all the guilds it must process. It will ingest a large block of messages before going to the next.

Within each channel, it will look for the earliest possible set of messages it can fetch in bulk, and then process those. It will do this by storing a range of snowflakes for each channel that records what parts of the history have been inserted already. It then updates the history record.

[emmiegit]

The first thing to implement is the Range class and similar, to represent abstract integer ranges for tracking which snowflakes have already been covered. See the range branch for info on this.

[emmiegit]

The range class has been completed and merged. Now the crawler proper (found on the crawler branch) needs to be completed.