Parallelize the server compute

Following up on this [comment](https://github.com/apple/swift-homomorphic-encryption/issues/45#issuecomment-2276367503). This would be really helpful for those trying to scale, as any performance improvements are valuable :) > Not today, though it should be relatively straightforward to parallelize the server compute in a few places: [here](https://github.com/apple/swift-homomorphic-encryption/blob/1cff02b950bfac8b572936169ceae8ef45e1af80/Sources/PrivateInformationRetrieval/MulPir.swift#L397-L398) [here](https://github.com/apple/swift-homomorphic-encryption/blob/1cff02b950bfac8b572936169ceae8ef45e1af80/Sources/PrivateInformationRetrieval/MulPir.swift#L351-L352) [here](https://github.com/apple/swift-homomorphic-encryption/blob/1cff02b950bfac8b572936169ceae8ef45e1af80/Sources/PrivateInformationRetrieval/PirUtil.swift#L128)