Threading refactor (4/7): Pool & op-executor contract #4

Job	Run time
bulk-filter	1m 5s
windows	2m 56s
lint	3m 19s
changes	1m 8s
units (8)	1m 45s
enforcer	21m 2s
gapic-libraries-bom	21m 17s
generation-config-check	1m 49s
units (11)	1m 16s
units (25)	1m 11s
units (17)	1m 15s
units (26)	1m 12s
units (21)	1m 28s
split-dependencies (java-bigtable)	6m 28s
split-clirr (java-bigtable)	3m 7s
split-units (java-bigtable, 8)	6m 29s
split-units (java-bigtable, 26)	8m 1s
split-units (java-bigtable, 11)	6m 37s
split-units (java-bigtable, 25)	6h 0m 15s
split-units (java-bigtable, 21)	8m 45s
split-units (java-bigtable, 17)	8m 31s
conditional-required-check	4s
	7h 49m 0s

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