basesink: Post a latency message whenever we're ready to answer the query

basesink: Post a latency message whenever we're ready to answer the query

Usually the latency message is only posted whenever latency of an
element changes but that might be too early as the sinks might not be
able to query the latency at that point yet.

Similarly adding a new sink should cause latency reconfiguration once
that new sink is able to report its latency.

This fixes latency configuration in pipelines where webrtcbin is the
only "sink", i.e. it is used in a sendonly session. Before, the latency
would always be configured to 0.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/843>