rpc-client: Use concurrency to avoid serial network calls

[joncinque]

Problem

The nonblocking RPC client has a tendency to serialize network calls unnecessarily, making the whole client slower than needed.

For example, see

agave/rpc-client/src/nonblocking/rpc_client.rs

Lines 683 to 694 in 64a8871

	let recent_blockhash = if transaction.uses_durable_nonce() {
	self.get_latest_blockhash_with_commitment(CommitmentConfig::processed())
	.await?
	.0
	} else {
	*transaction.get_recent_blockhash()
	};
	let signature = self
	.send_transaction_with_config(transaction, config)
	.await?;
	self.confirm_transaction_with_spinner(&signature, &recent_blockhash, commitment)
	.await?;

Proposed Solution

In that example, the first two network calls can happen concurrently, since they have no relation to each other. We should rather join! those two futures using https://docs.rs/futures/latest/futures/macro.join.html

And there are likely many more examples like this throughout the rpc-client.