Threading refactor (4/7): Pool & op-executor contract #23

Job	Run time
bulk-filter	36s
enforcer	21m 2s
lint	3m 28s
windows	2m 40s
changes	29s
generation-config-check	1m 49s
units (8)	1m 19s
units (11)	1m 13s
units (25)	1m 12s
gapic-libraries-bom	20m 31s
units (26)	1m 44s
units (21)	1m 23s
units (17)	1m 8s
split-clirr (java-bigtable)	3m 29s
split-units (java-bigtable, 8)	7m 8s
split-dependencies (java-bigtable)	6m 15s
split-units (java-bigtable, 17)	8m 57s
split-units (java-bigtable, 11)	6m 9s
split-units (java-bigtable, 26)	7m 30s
split-units (java-bigtable, 25)	7m 33s
split-units (java-bigtable, 21)	12m 2s
conditional-required-check	4s
	1h 57m 41s

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