Ensure that witness costs are charged per-tx and not per-block

[gballet]

The EIP definitions have changed, and the witness costs have to be charged per-transaction. We need to ensure that this is what the code does, because the old behavior was to charge said costs per-block.

[jwasinger]

In gballet/go-ethereum#37, I'm fairly sure this is the case. The AccessWitness used is per-TxContext (which as the name implies, only exists for the lifetime of a transaction).

[gballet]

Long fixed. Closing.