Add gap handling within bufsink

Add an is_gap property to Buffers returned from the buffer callback, while invoking the callback during gaps as well.

This allows a pipeline to handle gap cases when a buffer callback is added. For example, gstlal_inspiral does checkpointing and other processing even when all ifos are offline, and the conversion to the stream API introduced a regression here.

All pipelines that use bufsink have been updated accordingly:

• gstlal_inspiral
• gstlal_inspiral_calc_snr
• gstlal_ll_dq

The latter two effectively ignore the gap case, so we skip processing when we encounter gaps.