Improve transaction pool

[obscuren]

In order to prevent some DOS attacks we need to make the following changes to the block pool

• Check if sender account exist
• Check sufficient upfront payment
• Check intrinsic gas
• tx.nonce >= account.nonce

For now we'll allow the transaction nonce to be greater than the account nonce. This is easily upgradable if necessary.

[obscuren]

Implemented in 6184781