Remove the Blockstore thread pool used for fetching Entries #7739

Job	Run time
cargo-test-sbf (token/program)	7m 21s
cargo-test-sbf (instruction-padding/program, token/program-2022, token/program-2022-test)	12m 7s
check	0s
cargo-test-sbf (associated-token-account/program, associated-token-account/program-test)	8m 34s
test	0s
cargo-test-sbf (token-upgrade/program)	7m 28s
cargo-test-sbf (feature-proposal/program)	8m 41s
cargo-test-sbf (governance/addin-mock/program, governance/program)	12m 1s
cargo-test-sbf (memo/program)	7m 37s
cargo-test-sbf (name-service/program)	7m 21s
cargo-test-sbf (stake-pool/program)	12m 3s
cargo-test-sbf (single-pool/program)	8m 43s
	1h 31m 56s

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