Threading refactor (4/7): Pool & op-executor contract #15

Job	Run time
bulk-filter	1m 8s
enforcer	22m 42s
generation-config-check	1m 47s
units (8)	1m 27s
windows	2m 48s
changes	1m 9s
lint	3m 12s
units (21)	1m 11s
units (17)	1m 28s
gapic-libraries-bom	21m 26s
units (11)	1m 23s
units (26)	1m 27s
units (25)	1m 18s
split-clirr (java-bigtable)	2m 55s
split-dependencies (java-bigtable)	6m 10s
split-units (java-bigtable, 8)	6m 55s
split-units (java-bigtable, 26)	8m 32s
split-units (java-bigtable, 11)	7m 21s
split-units (java-bigtable, 25)	8m 39s
split-units (java-bigtable, 17)	9m 2s
split-units (java-bigtable, 21)	8m 7s
conditional-required-check	8s
	2h 0m 15s

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