Threading refactor (6/7): Tracer correctness & chain.isDone() #41

Job	Run time
bulk-filter	29s
lint	3m 6s
units (8)	2m 41s
changes	1m 8s
windows	2m 53s
enforcer	19m 48s
units (17)	1m 35s
generation-config-check	1m 54s
units (25)	1m 26s
units (21)	1m 31s
gapic-libraries-bom	21m 18s
units (26)	1m 24s
units (11)	1m 24s
split-units (java-bigtable, 8)	7m 10s
split-dependencies (java-bigtable)	6m 15s
split-clirr (java-bigtable)	3m 3s
split-units (java-bigtable, 26)	8m 40s
split-units (java-bigtable, 25)	13m 6s
split-units (java-bigtable, 21)	6m 31s
split-units (java-bigtable, 11)	6m 45s
split-units (java-bigtable, 17)	8m 46s
conditional-required-check	2s
	2h 0m 55s

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