chore: refactor concurrency model in the bgitable session protocol #22055

Job	Run time
bulk-filter	1m 18s
changes	27s
lint	11m 48s
windows	1m 59s
units (8)	1m 18s
enforcer	21m 13s
gapic-libraries-bom	21m 16s
generation-config-check	1m 9s
units (25)	41s
units (26)	44s
units (17)	54s
units (11)	45s
units (21)	52s
split-units (java-bigtable, 8)	6m 25s
split-dependencies (java-bigtable)	5m 17s
split-units (java-bigtable, 11)	6m 22s
split-units (java-bigtable, 21)	7m 44s
split-units (java-bigtable, 26)	7m 26s
split-units (java-bigtable, 25)	6h 0m 16s
split-clirr (java-bigtable)	2m 22s
split-units (java-bigtable, 17)	7m 40s
conditional-required-check	3s
	7h 47m 59s

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