decouple application buffer processing from aravis

[bmegli]

Implements #11 "thread per substream".

Fixes:

• 10

• as network thread is no longer stopped from working during processing

Each substream has processing thread for:

• GiGE-Vision/GenICam pixel format conversions to ROS
• ROS publishing

Technically:

• aravis buffers are delegated to relevant substream threads for processing.
• multithreading is achieved through signaling threads on new data + mutex guarded shared data
• buffer lifecycle ownership is achieved through shared_ptr
  • in case of multipart substreams all parts share original underlying buffer
  • ownership is passed to processing threads
  • when all threads are done, buffers return to aravis pool

Breaking Changes

Potentially PTP clock reset

• as it is code running from image/part processing handler and may not be thread safe