The current implementation only makes use of non-partial results,
requiring a crazy high latency.
With this mode, we use items from partial results when they're
older than latency - 2 * GRANULARITY_MS. Depending on the latency
that the user has set this may result in reduced accuracy, the
default latency has been modified to a pretty conservative sweet
spot of 8 seconds.
This complexifies the code a bit, as items aren't identified by
AWS, and their timings can change.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/-/merge_requests/348>
