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.

requested review from @rebecca.ewing

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• dac19912 - add gap handling within bufsink

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resolved all threads

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• dac19912...c5f3bc19 - 5 commits from branch master
• 0686c344 - add gap handling within bufsink

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• 7783c025 - lloidhandler.py: apply suggestions from Kipp

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added 19 commits

• 7783c025...90c64e5e - 17 commits from branch master
• 9208cf7a - add gap handling within bufsink
• a3a56c4f - lloidhandler.py: apply suggestions from Kipp

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merged