More validity checks for ClientTransactions

[ineiti]

A malicious client can send a lot of ClientTransactions that are invalid, but that will nevertheless be included in the chain. This makes it very easy to flood the chain with useless data.

Before a ClientTransaction is sent to the leader, there should be more checks:

• does it apply to the current state?

Caution must be taken when multiple ClientTransactions arrive that depend on each other, so as not to refuse valid transactions.