Threading refactor (5/7): Close ordering & shutdown hardening #28

Job	Run time
bulk-filter	29s
units (8)	1m 25s
lint	3m 11s
units (21)	1m 9s
generation-config-check	1m 50s
gapic-libraries-bom	14m 16s
enforcer	21m 49s
units (25)	1m 28s
units (26)	1m 25s
units (17)	1m 23s
changes	1m 9s
windows	3m 18s
units (11)	1m 27s
split-clirr (java-bigtable)	3m 16s
split-units (java-bigtable, 8)	6m 26s
split-dependencies (java-bigtable)	6m 22s
split-units (java-bigtable, 11)	7m 57s
split-units (java-bigtable, 26)	8m 23s
split-units (java-bigtable, 25)	8m 17s
split-units (java-bigtable, 21)	6h 0m 15s
split-units (java-bigtable, 17)	8m 12s
conditional-required-check	8s
	7h 43m 35s

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