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.