free the cluster cluster context if no longer used by any bdev #492

Job	Run time
build	7m 28s
build-ceph	2m 6s
discovery (container)	3m 56s
discovery (embedded)	9m 50s
demo	3m 33s
ha (sanity)	3m 45s
pytest (cli)	1m 52s
ha (state_transitions)	4m 11s
pytest (state)	1m 51s
pytest (multi_gateway)	2m 20s
ha (state_transitions_both_gws)	4m 38s
pytest (server)	2m 39s
ha (state_transitions_loop)	12m 34s
pytest (grpc)	2m 21s
ha (state_transitions_rand_loop)	14m 12s
pytest (omap_lock)	4m 31s
pytest (old_omap)	1m 59s
pytest (log_files)	3m 24s
pytest (nsid)	2m 46s
push-to-registry	0s
	1h 29m 56s

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