Fix propagation of signal metadata by DataSink

[frankosterfeld]

[This is part of fair-acc/opendigitizer#160 , to track graph-prototype-specific work, thus no separate SP]

Make sure that the signal metadata handling (sample_rate, signal_name, signal_unit, signal_min, signal_max) works correctly:
If the source has these properties and sends them down via tags (via the implicit propagation done for default tags), make sure they are picked up the data sink and forwarded in the respective DataSet fields (for triggered/multiplexed/snapshot) or tags (for streaming).
Make sure that property changes during runtime including signal_name are handled correctly and in a thread-safe manner.

[frankosterfeld]

Closing, tracked as fair-acc/opendigitizer#160