Threading refactor (6/7): Tracer correctness & chain.isDone() #50

Job	Run time
bulk-filter	1m 14s
changes	30s
lint	3m 31s
units (8)	1m 32s
enforcer	22m 36s
gapic-libraries-bom	20m 56s
windows	2m 51s
generation-config-check	1m 52s
units (17)	1m 16s
units (26)	1m 47s
units (25)	1m 15s
units (21)	1m 8s
units (11)	1m 11s
split-units (java-bigtable, 8)	6m 8s
split-dependencies (java-bigtable)	6m 2s
split-clirr (java-bigtable)	3m 28s
split-units (java-bigtable, 17)	9m 5s
split-units (java-bigtable, 11)	6m 58s
split-units (java-bigtable, 26)	8m 22s
split-units (java-bigtable, 21)	8m 32s
split-units (java-bigtable, 25)	8m 22s
conditional-required-check	3s
	1h 58m 39s

Provide feedback