replay: do not start leader for a block we already have shreds for #8555

Job	Run time
check	5m 8s
cargo-test-sbf (token/program)	8m 8s
test (token/cli, token/program, token/program-2022, associated-token-account/program, instruction...	15m 1s
cargo-test-sbf (instruction-padding/program, token/program-2022, token/program-2022-test)	16m 33s
test (single-pool/cli, single-pool/program)	18m 5s
cargo-test-sbf (associated-token-account/program, associated-token-account/program-test)	11m 6s
test (token-upgrade/cli, token-upgrade/program)	14m 4s
cargo-test-sbf (token-upgrade/program)	10m 31s
cargo-test-sbf (feature-proposal/program)	10m 28s
cargo-test-sbf (governance/addin-mock/program, governance/program)	14m 58s
cargo-test-sbf (memo/program)	6m 43s
cargo-test-sbf (name-service/program)	9m 5s
cargo-test-sbf (stake-pool/program)	11m 59s
cargo-test-sbf (single-pool/program)	11m 6s
	2h 42m 55s

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