background thread limit conclusion: 10 per backend
reconnect stopped twitter user stream connections
catch exceptions and continue when updating twitter streams
did a TODO
use normal threads for twitter streams instead of background threads

...since app engine backends have a fixed limit of 10 background threads per
instance. normal threads are only limited by memory, and since we're starting
them from a background thread, they're not bound to an HTTP request.
http://stackoverflow.com/a/20896720/186123
twitter_streaming: use runtime shutdown hook instead of /_ah/stop
more todos