Consume Bank in BankClient

This will allow BankClient to spin up a thread to use the Bank.
It'll also ease the transaction from BankClient to ThinClient since
it won't let you depend on Bank.

Drawback, you the transition from Bank to BankClient will be harder
because the Bank methods are inaccessible.

runtime/src/system_instruction_processor.rs

@@ -284,7 +284,7 @@ mod tests {
 
         // Fund to account to bypass AccountNotFound error
         let bank = Bank::new(&genesis_block);
-        let bank_client = BankClient::new(&bank);
+        let bank_client = BankClient::new(bank);
         bank_client
             .transfer(50, &alice_keypair, &mallory_pubkey)
             .unwrap();