fix (light/provider) : Make read_only executions read-only (#9591)

* `ExecutionsRequest` from light-clients as read-only

This changes so all `ExecutionRequests` from light-clients are executed
as read-only which the `virtual``flag == true ensures.

This boost up the current transaction to always succeed

Note, this only affects `eth_estimateGas` and `eth_call` AFAIK.

* grumbles(revert renaming) : TransactionProof

* grumbles(trace) : remove incorrect trace

* grumbles(state/prove_tx) : explicit `virt`

Remove the boolean flag to determine that a `state::prove_transaction`
whether it should be executed in a virtual context or not.

Because of that also rename the function to
`state::prove_transction_virtual` to make more clear

ethcore/src/spec/spec.rs

@@ -877,14 +877,13 @@ impl Spec {
 				data: d,
 			}.fake_sign(from);
 
-			let res = ::state::prove_transaction(
+			let res = ::state::prove_transaction_virtual(
 				db.as_hashdb_mut(),
 				*genesis.state_root(),
 				&tx,
 				self.engine.machine(),
 				&env_info,
 				factories.clone(),
-				true,
 			);
 
 			res.map(|(out, proof)| {