Parallelize transaction validation in prepare_transactions

[miloserdow]

Description

In the NightshadeRuntime implementation, the function prepare_transactions currently processes transactions sequentially. It would be great to parallelize at least validate_transaction calls here, potentially more.

// chain/chain/src/runtime/mod.rs
impl RuntimeAdapter for NightshadeRuntime {
// [...]
fn prepare_transactions(
        &self,
        storage_config: RuntimeStorageConfig,
        chunk: PrepareTransactionsChunkContext,
        prev_block: PrepareTransactionsBlockContext,
        transaction_groups: &mut dyn TransactionGroupIterator,
        chain_validate: &mut dyn FnMut(&SignedTransaction) -> bool,
        time_limit: Option<Duration>,
    ) -> Result<PreparedTransactions, Error> {
    // [...]
    // Take a single transaction from this transaction group
    while let Some(tx_peek) = transaction_group_iter.peek_next() {
        // [...]
        let res = validate_transaction(
                    runtime_config,
                    prev_block.next_gas_price,
                    &tx,
                    true,
                    protocol_version,
                );

See also: PR-12654