[TieredStorage] Make TieredStorage::write_accounts() thread-safe #7195

Job	Run time
check	0s
cargo-test-sbf (token/program)	7m 52s
test	0s
cargo-test-sbf (instruction-padding/program, token/program-2022, token/program-2022-test)	10m 34s
cargo-test-sbf (associated-token-account/program, associated-token-account/program-test)	7m 46s
cargo-test-sbf (token-upgrade/program)	6m 51s
cargo-test-sbf (feature-proposal/program)	7m 40s
cargo-test-sbf (governance/addin-mock/program, governance/program)	8m 56s
cargo-test-sbf (memo/program)	6m 32s
cargo-test-sbf (name-service/program)	6m 44s
cargo-test-sbf (stake-pool/program)	9m 22s
cargo-test-sbf (single-pool/program)	8m 16s
	1h 20m 33s

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