Main::PSGI: In _handle_tl_get_unacked_counts() and _handle_get_unacked_counts(), now future_of() is NOT used. Instead, Future::Q is manually created and used.
This is because if future_of() is used, $watcher will not be cancelled if the callback is called with error.
In addition, if the callback is called immediately, the future handlers are not called immediately because they are attached to the future AFTER returning from watch_* methods.
This can cause the callback being called multiple times when multiple timelines are watched via _handle_get_unacked_counts() and they fire immediately.
t/WebAPI.t: add test in which multiple weirdly-named timelines are watched by GET /updates/unacked_counts.json API.