Threading refactor (6/7): Tracer correctness & chain.isDone() #31

Job	Run time
bulk-filter	1m 9s
generation-config-check	1m 5s
units (8)	1m 7s
enforcer	20m 11s
windows	1m 57s
lint	3m 17s
changes	31s
units (26)	53s
gapic-libraries-bom	15m 14s
units (25)	50s
units (17)	53s
units (11)	45s
units (21)	1m 24s
split-units (java-bigtable, 8)	6m 46s
split-units (java-bigtable, 17)	8m 56s
split-dependencies (java-bigtable)	6m 18s
split-units (java-bigtable, 21)	8m 32s
split-units (java-bigtable, 25)	8m 44s
split-units (java-bigtable, 26)	8m 41s
split-units (java-bigtable, 11)	7m 21s
split-clirr (java-bigtable)	3m 14s
conditional-required-check	4s
	1h 47m 52s

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