streamer: Put streamer behind a feature flag

Only enable the "realtime" features if the "streamer" feature flag is
enabled.
streamer: Refactor annotation event handling

Move the handling of individual annotation events out of the callback
function and into a tested StreamClient class.
Add h.queue, for providing access to the queue

Adds two methods on the request:

- get_queue_reader
- get_queue_writer

which mediate access to the underlying NSQ queue.
WIP: decouple propagation of streamer events through a queue

This doesn't work yet, for reasons that aren't entirely clear (although
probably gevent related).

In principle this is the first step to propagating the "realtime
annotation" events through an external queue. More error-checking will
almost certainly be required, and sending the entire annotation through
the queue seems absurd... so WIP it is.