dataqueue: Make it understand buffers, segments, gaps

Submitted by Olivier Crête @ocrete

Link to original bug (#751918)

Description

In practice, every queue in GStreamer is buffers and synchronized events/queries, so it doesn't make sense to have a GstDataQueue that tries to abstract them, then re-implement the processing every it's used. As we integrate queues in more elements (like in demuxers), we can reduce duplication.