Threading refactor (4/7): Pool & op-executor contract #24

Job	Run time
bulk-filter	50s
lint	3m 21s
changes	1m 10s
enforcer	20m 15s
units (8)	1m 57s
windows	2m 29s
generation-config-check	1m 57s
gapic-libraries-bom	20m 30s
units (17)	40s
units (21)	1m 41s
units (25)	1m 26s
units (11)	1m 33s
units (26)	1m 27s
split-dependencies (java-bigtable)	5m 36s
split-clirr (java-bigtable)	3m 18s
split-units (java-bigtable, 8)	6m 44s
split-units (java-bigtable, 21)	8m 38s
split-units (java-bigtable, 26)	8m 7s
split-units (java-bigtable, 25)	7m 31s
split-units (java-bigtable, 17)	9m 1s
split-units (java-bigtable, 11)	6m 45s
conditional-required-check	2s
	1h 54m 58s

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