Multiple pending transactions with the same nonce should be allowed and properly prioritized

[davidmurdoch]

When interval mining it is possible two send multiple transactions with the same nonce value to be mined in a future block. Only the transaction with the greater gasPrice should actually be mined. The other transactions with lower gasPrice values should return error responses. When both the gasPrice and nonce are identical for transactions to be mined in a future block the transaction that arrived in the queue first should be mined and other transactions should be rejected.

geth's error response for this is {code:-32000,message:"replacement transaction underpriced"}.

[davidmurdoch]

done in [email protected]