Send only observation buffers that are in use. #9319

Job	Run time
examples-rl (e10_drive)	5m 22s
benchmark (e10_drive)	2m 0s
examples-rl (e11_platoon)	3m 0s
benchmark (e11_platoon)	1m 55s
base-tests (./cli)	2m 48s
base-tests (./envision)	2m 58s
base-tests (./smarts/core --nb-exec-timeout 65536 --ignore=./smarts/core/tests/test_notebook.py)	2m 58s
base-tests (./smarts/env --ignore=./smarts/env/tests/test_rllib_hiway_env.py)	2m 58s
base-tests (./smarts/env/tests/test_rllib_hiway_env.py)	2m 59s
base-tests (./smarts/sstudio)	2m 52s
base-tests (./smarts/ray)	2m 59s
base-tests (./examples/tests/test_examples.py)	2m 59s
	35m 48s

Provide feedback