timeline_list.js: add bb.UnackedCountsPoller class.
xt/js/unacked_counts_poller.html: add one more request sequence to the tests. Not so meaningful, though.
timeline_list.js: UnackedCountsRenderer: now it does not modify given unacked_counts object, so that it won't mess up with the UnackedCountsPoller. UnackedCountsPoller, on the other hand, pass the unacked_counts object to the callback by shallow-copying. This reduces the risk of being messed up with the unacked counts object.
timeline_list.tt: introduce bb.UnackedCountsPoller.