wen_restart: split bank_forks lock inside find_bankhash_of_heaviest_fork(). #8883

Job	Run time
cargo-test-sbf (token/program)	8m 28s
test (token/cli, token/program, token/program-2022, associated-token-account/program, instruction...	17m 22s
cargo-test-sbf (instruction-padding/program, token/program-2022, token/program-2022-test)	17m 1s
check	2m 14s
test (single-pool/cli, single-pool/program)	17m 28s
cargo-test-sbf (associated-token-account/program, associated-token-account/program-test)	9m 55s
test (token-upgrade/cli, token-upgrade/program)	17m 35s
cargo-test-sbf (token-upgrade/program)	7m 13s
cargo-test-sbf (feature-proposal/program)	8m 52s
cargo-test-sbf (governance/addin-mock/program, governance/program)	13m 4s
cargo-test-sbf (memo/program)	7m 17s
cargo-test-sbf (name-service/program)	7m 7s
cargo-test-sbf (stake-pool/program)	12m 33s
cargo-test-sbf (single-pool/program)	10m 10s
	2h 36m 19s

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