stream.py: add functionality to support gstlal_inspiral

* fold in state/DQ vector support in Stream.from_datasource
* expose setting pipeline state via stream.set_state(state)
* support operations on groups of keyed streamed via StreamMap
* allow callbacks via EOS messages where message types are not relevant
* support x-lal caps when pulling buffers from appsink
* support AppSync functionality for synchronization via StreamMap.bufsink