Changes db access and migrations as well as splitting into multiple processes

[chapel]

Lots of changes, I didn't mean to stack it all in one PR, but here we go.

• Only cache posts with a score > 10
• Move migrations to separate migration folder with commands to run them
• Split cron, feed, stream scripts into separate processes that run at the same time
  • This is to avoid the forced sync nature of some db actions, e.g. cron job cache read is slow and blocks everything in the same process
• Move to async db access wherever we can, it isn't the most useful since some of the query commands are still sync

[chapel]

Note if you deploy this, the initial delete there is going to take some time. Not sure how best to go about it, if you're okay with a short bit of downtime, you could manually run it against the db.

Edit: added a migration that you can run while the server is still running so you can prune low score posts without taking everything down. Though as well the split process model might mitigate the slowness/issue with a large delete like that.

[hipstersmoothie]

@chapel will deploy during the night

[chapel]

I will say, it seems to have been working well all this time, not sure how big the databases are now, but from an end user perspective the feeds have felt generally snappy.