Only return latest hashes when responding to live channel time range requests

[mycognosist]

Responses to live channel time range requests are currently rather inefficient, since they send hashes for all posts matching the channel options every time a new post is published. It would be much better to only send a singe hash each time a new post is published.

The relevant code can be found in the send_post_hashes() method, specifically in the match arm for LiveRequest::ChannelTimeRange(req_id, channel_opts).