Blockstore: Migrate ShredIndex type to more efficient data structure #16028

Job	Run time
cargo-test-sbf (token/program)	7m 3s
cargo-test-sbf (instruction-padding/program, token/program-2022, token/program-2022-test)	12m 30s
cargo-test-sbf (associated-token-account/program, associated-token-account/program-test)	8m 33s
cargo-test-sbf (token-upgrade/program)	7m 58s
check	2m 53s
cargo-test-sbf (feature-proposal/program)	7m 5s
test (token/cli, token/program, token/program-2022, associated-token-account/program, instruction...	15m 41s
cargo-test-sbf (governance/addin-mock/program, governance/program)	10m 38s
test (single-pool/cli, single-pool/program)	14m 45s
cargo-test-sbf (name-service/program)	7m 39s
test (token-upgrade/cli, token-upgrade/program)	13m 50s
cargo-test-sbf (stake-pool/program)	10m 31s
cargo-test-sbf (single-pool/program)	7m 58s
	2h 7m 4s

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