message: rename GST_MESSAGE_DURATION -> GST_MESSAGE_DURATION_CHANGED

message: rename GST_MESSAGE_DURATION -> GST_MESSAGE_DURATION_CHANGED

The duration should be re-queried via a query using the
normal path, we don't want applications to use the value
from the message itself, since it might no match what a
duration query done from the sink upstream might yield.

Also disables duration caching in GstBin. It should be
added back again at some point.