Concurrent-ify RSS HTTP requests

[jesopo]

BitBot suffers noticeable minutely lag when he's off checking a bunch of RSS feeds because it is done in-serial - if each request takes half a second and there's 20 urls it's going to take longer than is acceptable

I propose making this work by using async http requests (I've laid the groundwork here 15e143f)

[jesopo]

done as of c19c6c0