Add hidden CLI arg to control number of shred sigverify threads (#3046) #490

Job	Run time
cargo-test-sbf (token/program)	9m 44s
cargo-test-sbf (instruction-padding/program, token/program-2022, token/program-2022-test)	16m 21s
check	3m 11s
cargo-test-sbf (associated-token-account/program, associated-token-account/program-test)	10m 5s
cargo-test-sbf (token-upgrade/program)	9m 6s
test (token/cli, token/program, token/program-2022, associated-token-account/program, instruction...	19m 32s
cargo-test-sbf (feature-proposal/program)	9m 45s
test (single-pool/cli, single-pool/program)	19m 41s
cargo-test-sbf (governance/addin-mock/program, governance/program)	12m 23s
test (token-upgrade/cli, token-upgrade/program)	18m 11s
cargo-test-sbf (memo/program)	8m 2s
cargo-test-sbf (name-service/program)	10m 22s
cargo-test-sbf (stake-pool/program)	10m 44s
cargo-test-sbf (single-pool/program)	9m 35s
	2h 46m 42s

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