Enusre pending state is ready for tx execution

This commit ensures that StateDB.Prepare is called on the pending state
before it is returned from the miner/worker. This prevents access logs
from previous transaction executions from interfering with the gas cost
calculations of the subsequently executed transaction.

The problem this solves was introdued by this upstream PR
ethereum/go-ethereum#21509 which added an
access list to the state which was used to reduce gas costs for repeated
access of the same state locations, this resulted in the pending block
having the access list of the last executed transaction, which could
cause gas estimates to be wrong when the estimated transaction accessed
some of the same state as the prior transaction.

miner/worker.go

@@ -168,7 +168,12 @@ func (w *worker) pending() (*types.Block, *state.StateDB) {
 	if w.snapshotState == nil {
 		return nil, nil
 	}
-	return w.snapshotBlock, w.snapshotState.Copy()
+	stateCopy := w.snapshotState.Copy()
+	// Call Prepare to ensure that any access logs from the last executed
+	// transaction have been erased.
+	// See https://github.com/celo-org/celo-blockchain/pull/1858#issuecomment-1054159493
+	stateCopy.Prepare(common.Hash{}, 0)
+	return w.snapshotBlock, stateCopy
 }
 
 // pendingBlock returns pending block.