[TieredStorage] Refactor TieredStorage::new_readonly() code path (#195) #89

Job	Run time
check	0s
cargo-test-sbf (token/program)	8m 16s
test	0s
cargo-test-sbf (instruction-padding/program, token/program-2022, token/program-2022-test)	13m 33s
cargo-test-sbf (associated-token-account/program, associated-token-account/program-test)	10m 23s
cargo-test-sbf (token-upgrade/program)	5m 54s
cargo-test-sbf (feature-proposal/program)	6m 51s
cargo-test-sbf (governance/addin-mock/program, governance/program)	11m 52s
cargo-test-sbf (memo/program)	8m 43s
cargo-test-sbf (name-service/program)	7m 45s
cargo-test-sbf (stake-pool/program)	9m 36s
cargo-test-sbf (single-pool/program)	10m 59s
	1h 33m 52s

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