Fix/same nonce replacement txs

[MicaiahReid]

Prevents a transaction that replaces a previously queued transaction from being re-added to the transaction pool and thus run twice.

Previously when a transaction was sent we checked:

• do we have executable transactions in the transaction pool from the same origin?
• does this new transaction have the same nonce as one of those?
• if so, is this new transaction's gas price better?
• if so, replace the previous transaction with this new one.

We now perform all of those same checks against the future-nonce transactions (transactions whose nonce is higher than the sender account's nonce) in the transaction pool, not just the executable transactions.

Fixes #1219 and #1218

[davidmurdoch]

Looks like tests aren't completing right now. See the removes transactions that are in pending transactions at the start of evm_revert test. It passes, but then mocha just quits. This seems to happen when Node.js has determined that the application has hit a dead end and thus no more events will be occurring. Let me know if you want to sync on this one!