Threading refactor (6/7): Tracer correctness & chain.isDone() #27

Job	Run time
bulk-filter	58s
changes	1m 6s
gapic-libraries-bom	21m 44s
windows	2m 28s
units (8)	1m 26s
generation-config-check	1m 40s
units (21)	1m 10s
enforcer	20m 8s
lint	3m 7s
units (17)	1m 9s
units (11)	1m 15s
units (26)	1m 12s
units (25)	1m 18s
split-clirr (java-bigtable)	3m 6s
split-units (java-bigtable, 8)	6m 43s
split-dependencies (java-bigtable)	6m 26s
split-units (java-bigtable, 25)	8m 13s
split-units (java-bigtable, 26)	8m 37s
split-units (java-bigtable, 17)	8m 53s
split-units (java-bigtable, 21)	8m 56s
split-units (java-bigtable, 11)	7m 4s
conditional-required-check	2s
	1h 56m 41s

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