separates durable nonce and blockhash domains

AdvanceNonceAccount instruction updates nonce to blockhash. This makes it
possible that a durable transaction is executed twice both as a normal
transaction and a nonce transaction if it uses blockhash (as opposed to nonce)
for its recent_blockhash field.

The commit prevents this double execution by separating nonce and blockhash
domains; when advancing nonce account, blockhash is hashed with a fixed string.
As a result a blockhash cannot be a valid nonce value; and if transaction was
once executed as a normal transaction it cannot be re-executed as a durable
transaction again and vice-versa.

interface/src/instruction.rs

@@ -736,7 +736,7 @@ pub fn create_nonce_account(
 ///     # });
 ///     let nonce_account = client.get_account(nonce_account_pubkey)?;
 ///     let nonce_data = nonce_utils::data_from_account(&nonce_account)?;
-///     let blockhash = nonce_data.blockhash;
+///     let blockhash = nonce_data.blockhash();
 ///
 ///     tx.try_sign(&[payer], blockhash)?;
 ///